If you’ve read more than one review on an aquarium heater, you’ve probably seen horror stories of electrical shocks, tanks hitting 120F, fish burns, or even melting and exploding heaters. Which might have you asking things like, which heaters are safe? How do I know if they’re safe? And – most importantly – how do I make sure my fish don’t get killed?
Unfortunately, heaters and the risks they come with are a necessary evil when keeping tropical fish. There aren’t any 100% foolproof methods to completely remove the chances of frying your fish from the equation if you’re going to use a heater. And, statistically, the longer you keep fish for, the more likely something like this will happen.
But there are a few precautions you can take to reduce the risks. The biggest one being redundancy. If your fish are sensitive to cold water, buy two lower wattage heaters (in the 3WPG range.) If you’re worried about your water overheating, hook your heater up to an alert system – which aren’t as expensive as you think. Use multiple thermometers when calibrating your heater and keep multiple thermometers in your tank to monitor it. Redundancy goes a long way in ensuring your fish’s safety.
The second thing is buying a quality heater. Yes, they’re usually more expensive, but it’s worth the price when you consider how much safer they are and how much longer they last. If you’re looking for some quick picks, I have the best products below. But if you’re looking for a more detailed review as well as some additional information about the different types of heaters, how to tell if you should trash your heater, how to calibrate your heater, and how to avoid breaking your heater, read on!
Disclosure: we’re reader-supported! So if you buy a product I recommend, I might make some coffee money at no cost to you.
Table of contents
Table of Contents
Table Of Contents
BEST HEATER
Aqueon Pro Submersible
The lifetime warranty, shatter-resistant housing, cheap price point, and ease of calibration make this one a no-brainer for me. Chewy’s track-record is better than Amazon’s with this one, so you might want to consider buying it from there – but it’s your choice. If you’re struggling to keep it calibrated (which is the most common issue) there are some great external controllers in this list too.
BEST ALARM
Inkbird C929A
This is one of the few Inkbird products made specifically for aquariums and, as such, it has some cool features. Of course, you have your normal high and low temp alerts, but you also have an alert for the heater run time that you can set. It also comes with probes that are – YAY! – made for aquariums and two of them to boot. Additionally, it has app notifications, a sound alarm, monitoring from the app, and a double relay on a single plug – also for redundancy.
It’s redundant in all the best ways – and it’s affordable!
BEST THERMOMETER
Zacro Digital Thermometer
Accurate, reliable, affordable, and has a long battery life. I’ve had mine for years and never had to change the battery. The suction cup isn’t great though, and you might be able to beat the (already low) price by looking for the same model branded by a different company.
How Many Watts Do I Need?
A general rule is 3 – 5 watts per gallon is good (or roughly 1 watt per liter for our metric friends.) Which is helpful, but how many watts you need isn’t just about how many gallons you’re trying to heat. Your room temp plays a critical role in how many watts you need as well. The math is simple, but a little difficult to remember given the variables.
With the exception of anything under a 10-gallon or any tank without a lid, you can multiply the gallons by 3 watts to get the water temp 5F or less above room temp. Multiply by 4 watts to get the water temp 10F or less above room temp. To get the water temp 15F above room temp, multiply by 5. For every 5 degree increment thereafter, you can add an additional watt per gallon.
For example, 20 above room temp would be 6 watts per gallon (6WPG), 25 above would be 7WPG, etc. If you don’t have a lid, add an additional WPG for every +5F above room temperature, and if your tank is smaller than 10 gallons, stop at 5WPG for safety.
Confused? That’s okay, I made a chart.
| +5F Room Temp (3WPG) | +10F Room Temp (4WPG) | +15F Room Temp (5WPG) | +20F Room Temp (6WPG) | |
|---|---|---|---|---|
| 10-Gallon | 30w | 40w | 50w | 60w |
| 20-Gallon | 60w | 80w | 100w | 120w |
| 30-Gallon | 90w | 120w | 150w | 180w |
| 40-Gallon | 120w | 160w | 200w | 240w |
| 50-Gallon | 150w | 200w | 250w | 300w |
| 60-Gallon | 180w | 240w | 300w | 360W |
| 75-Gallon* | 225w | 300w | 375w | 450w |
| 90-Gallon* | 270w | 360w | 450w | 540w |
*For larger tanks: if you need to heat your tank up more than 20F above room temp or have an extremely large tank, you might want to look into splitting the wattage between heaters. For example, instead of one 630 watt heater, you could do two 300 watt or four 150 watt heaters. One large in-line heater would probably be more efficient for tanks that size though.
Preset Versus Adjustable
You can find most of the heater types below in preset or adjustable. The difference between the two is, of course, one you can adjust the temperature and the other you can’t. Presets usually have a wider range of temperatures they fluctuate between, 76 – 80 being the standard. Because of this, presets are usually the cheaper option, but they tend to have calibration and overheating issues.
Best Preset Aquarium Heaters
Because of the horror stories of fish being cooked by presets, all the heaters I would recommend are adjustable. If you have a preset, you can convert it to an adjustable by buying an external control for it, which may be cheaper than replacing the heater altogether. But if you really want a preset, here are some of the best options (though, again, I recommend buying an adjustable heater):
Aqueon Preset Heater
Rating: 4.2 stars
Price: $8 – $14 *size dependent
Depending on the size, you can heat anything from a 20-gallon to a 75. This heater is preset to 78 F (26 C), but if your room temp is lower than 72 F, it likely won’t get that high. The main issue seems to be underheating here, which I’ve never personally had an issue with.
Pros:
- Fully submersible
- Shatter-resistant
- Fresh, brackish, and saltwater safe
Cons:
- Preset
- Issues with underheating
Marina Compact Heater
Rating: 4 stars
Price: $10 – $17 *size dependent
This heater could heat anything up to a 2.5-gallon. The temperature is preset to 78 F (26 C), but only if the room temperature is 72 F or higher. If it’s lower than that, your tank temperature will likely be lower. I’ve never experienced electrical issues, but more than one person has reported they can feel it.
Pros:
- Fully submersible
- Fresh, brackish, and saltwater safe
- Small and slim
Cons:
- Preset
- Reported electrical issues (zapping)
- Calibration issues (and you can’t recalibrate)
Tetra HT Submersible
Rating: 4 stars
Price: $11 – $12 *size dependent
Depending on the size, you can heat anything from a 2-gallon to a 30-gallon tank. I’ve used these heaters for years, but when they go, they cook! This heater is preset to 78 F (26 C), but if your room temp is lower than 72 F, it likely won’t get that high.
Pros:
- Fully submersible
- Fresh, brackish, and saltwater safe
Cons:
- Preset
- Issues with overheating
Types Of Internal Aquarium Heaters
Internal heaters go inside your tank, versus external which go outside your tank. Depending on which type of internal heater you go with, it can be a little more difficult to hide them, but they’re generally easier for most people to find, less expensive, and faster to set up. Additionally, if you have a sump, you can hide these guys in the sump in most cases.
Submersible
Good for: non-show tanks
Submersible heaters can – and should be – fully submerged in the water. They usually need to be placed sideways in the middle of the tank. This makes them one of the most difficult heaters to hide. If they’re placed upright in the tank, most models will shut off before the tank is heated since the hot water will rise to the thermostat and effectively trick the heater into shutting off.
Since this heater is sideways, it makes a tempting place for fish to perch on, which can lead to burns. If your fish perches on your heater, you can get a heater guard and this will lessen the risk of burns.
Pros:
- Waterproof
- Fresh, brackish, and saltwater safe
- Heats water more evenly
Cons:
- More difficult to hide
- Harder to access controls
- Easier for fish to burn on
Best Submersible Heaters
Aqueon Pro
Rating: 4 stars
Price: $22 – $31 *size dependent
Since they changed to a new manufacturer, these heaters can be hit or miss – but Chewy seems to have a better track record than Amazon. My heaters from 2014 are still going strong, but even the new heaters come with a lifetime warranty.
Pros:
- 68 F – 88 F
- Lifetime warranty
- Not glass
- Shatter-resistant
Cons:
- Needs calibrating out of the box
- Dial can be fiddly
- Bulky for smaller tanks
Hygger Titanium
Rating: 4.5 stars
Price: $50 – $93 *size dependent
This heater comes with an external controller, a wide range of temps, is shatterproof, and has great reviews. I didn’t give it the top spot because is there’s no ability to hook it up to another controller for redundancy – making failures in the unit’s controller catastrophic -, the warranty is too short for me, and there’s no way to recalibrate it.
Pros:
- 32 F – 104 F
- External controls
- Shatterproof
Cons:
- No redundancy option
- No recalibration option
- Only 1-year warranty
Cobalt Neo-Therm
Rating: 3.9 stars
Price: $56 – $106 *size dependent
Cobalt Neo Therms are some of the most reliable heaters on the market. There does seem to be some issues with certain wattages, however, and overheating seems to be a consistent issue for those models (150w & 200w being the most common.)
Pros:
- 66 F – 96 F
- 0.5-degree accuracy
- Super sleek
- Shatter-resistant
Cons:
- Overheating issue
- Only a 3-year warranty
- Expensive
Eheim Jager
Rating: 3.9 stars
Price: $20 – $32 *size dependent
Eheim is known for their heaters, and the Jager is one of the more affordable models they make. It does a great job of holding temp once it settles, but the built-in calibration has been an issue for some, particularly at low temps.
Pros:
- Built-in calibration
- 65F – 93F range
- Dry run safety shutoff
- Shatter-resistant
Cons:
- Calibration is fiddly
- Only a 3-year warranty
- Really long/difficult to hide
Immersible/Hanging
Good for: when you have literally no other option… for some reason
Immersible heaters cannot be fully submerged in water. The upper portion of the heater needs to remain out of the water since it’s not watertight and the wiring isn’t waterproof. It’s also not suitable for brackish or saltwater tanks since salt can spray into the circuitry. Oh, also these heaters are expensive and can electrocute you or your fish.
In short: you can do better for less money. But if you really want one, I have the best options below.
Pros:
- Vertical positioning makes it easier to hide
- Easier to access controls
Cons:
- Sticks out of lid
- Could short out if falls into water
- Could electrocute fish if falls into water
- Not salt or brackish safe
- Expensive
Best Immersible/Hanging Heaters
Finnex Deluxe
Rating: 3.9 stars
Price: $44 – $56 *size dependent
The price above doesn’t include the controller – which you need to run it – because they don’t sell that kit at all wattages. You can, however, connect it to a non-Finnex controller/alarm system, which most people chose to do.
Pros:
- Basically indestructible
- Can use preferred external controller or alarm system
Cons:
- Only large wattages
- Shock/shortage issues
- No redundancy options
JBJ True Titanium Heater
Rating: 3.1 stars
Price: $102 – $168 *size dependent
The biggest complaint is that the probe needs cleaning every 3 months and needs to be replaced every 8 – 12 months. Which the company doesn’t say anything about publicly. So every year you’ll need to contact them and spend at least $20 to replace the part.
Pros:
- Basically indestructible
- Easy to access controls
Cons:
- Shady practices on company’s end
- Consistent replacement parts
- No way to recalibrate
Heater Cables/Substrate Heating
Good for: aquascapes
Heater cables are difficult to get if you live in the US unless you can find a place to ship them internationally. Also, these cables won’t be able to heat your whole tank, they just provide localized heat to your substrate. This heat aerates your substrate, reducing the risk of anaerobic pockets. It also makes nutrients available to your plants faster and sooner.
In most cases, this heating solution will fix the “rip your tank apart every year” problem most aquascapers have. Some of the heat from your substrate will eventually make its way into your tank’s upper water columns but, again, this is not a “heat your whole tank” solution. Additionally, unless you’re using an external control, there’s usually no way to control the temperature output of these cables, they will just always be on.
Pros:
- Easy to hide
- Great for plants
- Reduces planted tank upkeep
Cons:
- Difficult to add to existing setup
- Can be hard to control
- Not a whole tank heating solution
- Can be hard to get in the states
Best Substrate Heaters
Hydor Hydrokable Series
Rating: 4 stars
Price: Currency conversion dependent
Hydor also has a good track record with substrate heaters but because you’ll need to ship internationally, be prepared to do some conversions and have an international to US plug converter handy. But you can at least expect the directions to be written in English.
Pros:
- Fully submersible
- Fresh, brackish, and saltwater safe
- Great for plants
Cons:
- Not adjustable
- Difficult to get
- Conversion issues
JBL Protemp Series
Rating: 4 stars
Price: Currency conversion dependent
JBL has a good track record with substrate heaters but because you’ll need to ship internationally, be prepared to do some conversions and have an international to US plug converter handy. Also, don’t expect the instruction to be written in proper English.
Pros:
- Fully submersible
- Fresh, brackish, and saltwater safe
- Great for plants
Cons:
- Not adjustable
- Difficult to get
- Conversion issues
Types Of External Heaters
External heaters are a little bit more advanced for the beginner fish keeper, but they have their benefits. Of course, the heater being out of the tank is a big one – fish can’t burn themselves and you don’t have to hide the heater in the tank.
In-Line Heaters
Good for: boisterous fish/closed rack systems
In-line heaters either need to be fitted onto an existing external filtration system or need to have an independent pump to circulate water from your tank, through the heater, and back into the tank. Depending on your setup and skill level, this could get complicated and leaky due to the additional pipework and fittings needed to make this work.
Pros:
- No fish burns
- Better heat circulation
- Easy to hide
Cons:
- Some DIY skill/extra parts needed
- Regular cleaning needed
- Risk of leaks
Best In-Line Heaters
ISTA In-Line Heater
Rating: 3.8 stars
Price: $53 – $100 *size dependent
ISTA runs from 150w – 500w, depending on the size and comes in two fitting sizes; 1/2″ and 5/8″. Though the biggest complaint seems to be that the fittings don’t fit as advertised and you may need to troubleshoot the fitting before it matches correctly.
Pros:
- Super stable temps
- Fresh, brackish, and saltwater safe
- No fish burns
Cons:
- Leaking is an issue
- Fitting size is an issue
Hydor In-Line
Rating: 3.8 stars
Price: $55 – $60 *size dependent
The biggest issue with this was it breaking in the “on” position and overheating. This is an easy fix with an external controller, but it’s less than ideal. Additionally, it only comes in 150w+, but it’s one of the cheaper in-line heaters.
Pros:
- No fish burns
- Fresh, brackish, and saltwater safe
Cons:
- Can leak
- Issues with overheating
- Only compatible with sumps & canisters
Thermofilters
Good for: non-DIYers
They’re difficult to find, and there’s really only two on the market so far, but they take the concept of a sump and throw it into a canister filter format. If you’re not familiar with either of those terms, it means they added a submersible heater into an external filter (to oversimplify things.) This gives you the benefits of an in-line heater, without the risk of leaks from the additional plumbing, but none of the flexibility of a sump for a lot more money.
Pros:
- Easy to hide
- No fish burns
- Low leak risk/no DIY needed
- Compact
Cons:
- Expensive
- Only a few products on the market
- If the heater fails you might need to replace the whole thing
Best Thermofilters
OASE Indoor Biomaster Thermo
Rating: 4 stars
Price: $230 – $310 *size dependent
Includes mechanical, biological, and chemical filtration as well as a prefilter and heating unit in a compact design. Also included is a 3-year warranty. BUT the heater in the unit is rated for a tank much smaller than the filter is rated for.
Pros:
- Compact
- Multi-type filtration
Cons:
- Expensive
- Not great for heating
- Reportedly purges often
Eheim Pro Thermofilter
Rating: 4 stars
Price: $512 *size dependent
I love most Eheim products, and they work great – but the second they stop working you’re alone. Eheim’s support is less than spectacular. Additionally, some of their sizes can be difficult to locate and most of the time don’t work with international shipping.
Pros:
- Quiet and compact
- Heater seems to function correctly
- No leakage issues
Cons:
- Eheim has terrible customer support
- Seriously, it’s horrible
- Super expensive
Heating Tape
Good for: lots of small bare bottom tanks
Heating tape is great for heating a ton of smaller tanks but doesn’t work great for heating large tanks. It’s also, assuming you have a dedicated fish room, not as cheap as heating the whole room. Most of the time you’ll see this in betta breeding or killifish setups where you need a ton of small tanks with minimal water movement.
You’ll also need some DIY electrical experience – though not much – as well as some additional fittings, clips, wires, and probably an external control. Below I have the only heated tape that I’m aware of on the market for fish tank as well as some of the accessories you’ll need, but you might also need dielectric grease and electrical tape.
Pros:
- Economic for multi-tank setups
- Can be easy to hide
Cons:
- DIY skills/extra parts needed
- Not suitable for anything over a 10-gallon
- Not suitable with substrate
- Not the most efficient heat
Flexwatt Heat Tape
Rating: 4.5 stars
Price: $5.90 per foot
I suggest buying Flexwatt from their website so you know you’re getting the real thing. With Amazon, it can be a little tricky to judge the authenticity. If you don’t already have the insulators and wire, you’ll need those too (below.)
Complete Connector Set
Rating: 4.5 stars
Price: $14
Comes with the wire and connectors (pictured) as well as a full set of insulators (not pictured.) You’ll need one set for every “line” of heat tape you intend to have.
If this seems like a foreign concept to you, that’s okay, here’s a video to give you ideas on how to set it up:
Heater Accessories
You likely won’t need all of these accessories for your heater, but it’s nice to know they exist if you do find you need them in the future. Bare minimum, you’ll need thermometers to calibrate your heater.
Best Aquarium Thermometers
Thermometers are, obviously, important for aquariums. You’ll need them to calibrate your heater as well and make sure your heater continues to function properly. Additionally, if you ever want to get an alert system or external control, you’ll need your trusted thermometer to calibrate those as well.
I suggest buying at least two, just to make sure they’re close to each other and functioning properly. I suggest digital thermometers, just because there’s one number versus a range you need to guestimate.
Zacro Digital Thermometer
Rating: 4.4 stars
Price: $7
These things are super accurate, the batteries last forever, and they’re cheap – but they’re not necessarily by Zacro. They seem to be one of those white-label products you can buy and rebrand, so most of the ones that look like this work well. However, the suction cups are always junk.
Pros:
- Accurate
- Cheap
- Batteries last a long time
- F/C
Cons:
- Suction cups don’t work – like ever
Capetsma Digital Thermometer
Rating: 4.4 stars
Price: $17 – $18 *color-dependent
Doesn’t work on acrylic tanks, but you can set the “high” and “low” temps so the thermometer will flash when it goes out of those parameters. It does have a shorter battery life than most thermometers though.
Pros:
- Touch screen
- Large print
- Can set “alert”
Cons:
- Short battery life
Capetsma Thermometer with Alarm
Rating: 4 stars
Price: $15
This looks like another white-label thermometer, but this one comes with a sound alert when the temperature gets out of the accepted range. You can set the high and low temps to whatever you want, and the batteries last a long time. However, the accuracy is hit or miss and the sound isn’t loud when it goes off.
Pros:
- Comes with alarm
- Batteries last a long time
- Easy to set & read
Cons:
- Shotty instructions
- Alarm isn’t loud
- Debatable accuracy
Best Heater Guards
Heater guards are made to protect your fish from your heater and – in some cases – protect your heater from your fish. If you buy one, you’ll likely need to clean your heater and the guard monthly to prevent any organic debris from building up and burning on the heater.
There aren’t a ton of heater guards on the market, but this one works well for most models and you can make it smaller if you need to.
uXcell Heater Shield
Rating: 4.2 stars
Price: $9
Depending on the size of your heater, you may be able to get this to work. It comes with breakaway sections so you can make it smaller to fit shorter heaters.
You can, if you’re so inclined, decide to DIY your own to fit your needs. Though this isn’t always cheaper, they usually work better for your setup, fit correctly, and look better as well.
Best External Heater Controls
External heating controls are great for redundancy as well as ease of access – particularly if your heater is in a tough spot to get to. The hitch is because they’re convenient to tinker with, if you have small children or wily pets, they could change the settings on you.
In some cases, you can hook up a preset heater to external control and turn the heating unit into an adjustable one. This works well with some cheaper preset heaters as well as the heating tape. Though I suggest most people who are concerned about the accuracy of their heaters purchase one of these for peace of mind.
Inkbird ITC-308
Rating: 4.6 stars
Price: $35
This model comes with a heating and cooling plug that are controlled independently by a dual relay. There is a buzzing alarm that will go off if the temperature exceeds the high or low you set or if there is an issue with the sensor itself.
Pros:
- Waterproof
- Dual relay
- Reliable & accurate
Cons:
- Needs probe protector for longterm aquarium use
Inkbird ITC-608T
Rating: 4.5 stars
Price: $55
This one comes with two plugs and the option to buy a second temperature probe so you could run two aquariums with one unit. This temperature sensor is fully waterproof, and it also has an alarm system you can turn off if you want.
Pros:
- Waterproof
- Accurate
- Buzz alarm
Cons:
- Some issues with the single relay system
Bayite Controller BTC201
Rating: 4.4 stars
Price: $30
There’s not much that sets this one apart from the Inkbirds above aside from the slightly cheaper price point and slightly worse reviews.
Pros:
- Waterproof
- Heating & cooling functions
Cons:
- May need to buy probe protector for longterm aquarium use
You can also DIY one of these if you have the time and skill.
Best Heater Alarm Systems
Most alarm systems come with a temperature probe and connect to your electronics so it can alert you even if you’re away from home. If you’re concerned about losing your fish, this is the best option to make sure they stay safe. Most, though not all, also come with external controls you can override remotely. And most of these kits really aren’t as expensive as you’d think they are since they’re just external controllers with Wifi capabilities.
Inkbird Wifi C929A
Rating: 4.6 stars
Price: $45
This is one of the few Inkbird products made specifically for aquariums and, as such, it has some cool features. Of course, you have your normal high and low temp alerts, but you also have an alert for the heater run time. It also comes with probes that are – YAY! – made for aquariums and two of them for redundancy. Additionally, it has app notifications and a double relay on a single plug, also for redundancy.
Pros:
- Redundant in the best way
- Specifically made for aquariums
- Two probes/two relays/heater run time alert
Cons:
- Temp reportedly swings 0.7F
Inkbird Wifi ITC-308
Rating: 4.6 stars
Price: $50
This model comes with a heating and cooling plug that are controlled independently by a dual relay. There is an alert that will go off if the temperature exceeds the high or low you set or if there is an issue with the sensor itself. It also comes with an app for monitoring it since it’s Wifi capable.
Pros:
- Waterproof
- Dual relay
- Reliable & accurate
Cons:
- Needs probe protector for longterm aquarium use
WILLHI 1436A-WIFI
Rating: 4.3 stars
Price: $50
This model comes with a single plug that can run heating or cooling functions as well as a circuit breaker to prevent surges. There is an alert that will go off if the temperature exceeds the high or low you set or if there is an issue with the sensor itself. It also comes with an app for monitoring it since it’s Wifi capable.
Pros:
- Waterproof
- Reliable & accurate
Cons:
- Needs probe protector for longterm
- Issues connecting to Wifiaquarium use
Aquarium Heater FAQ
Should My Heater Have Condensation?
Unless the packaging says something specifically about this, the answer is no. This means your heater is no longer watertight (or it never was) and its internal components are compromised. It’s time to switch your heater off and replace it ASAP.
When Should I Replace My Heater?
If your heater is over or underheating your fish tank, it’s time to replace it. Other good indications that it’s time to part ways are:
- You can see cracks (or if you’ve heard it crack or pop – not click*)
- You’ve “smoked it” (it will literally smoke)
- There are burn marks on the heater
- Condensation inside the heater
- Sounds like there’s something rattling around in there
- Buzzing or thumping sounds when operating or off*
- Repeatedly turns on and off rapidly (make sure it’s in the right position before you junk it though!)
* Heaters usually make a click sound when they turn on or off, that’s normal.
How Do I Calibrate My Heater?
Your heater should come with instructions for calibrating that specific model. In case you lost them or they didn’t make sense to you, the general process is about the same for every heater:
- Grab a 5-gallon bucket of water and two thermometers that you trust. (Recommendations at the bottom of this article if you don’t know where to start.) I suggest two so you can average the temperatures if they’re off from one another.
- Set your heater to the desired temperature and place it in the bucket with the thermometers. Wait for the heater to turn off or the temperature to stabilize. If you want to do this in your tank, you can, just be sure you keep an eye on it and know that it will take longer for it to heat up.
- Check to see if your thermometers match your heater setting. If they do, you’re all set! If not, you can calibrate it one of two ways. If your heater has a calibration ring, like the Eheim Jager, then you can move the calibration ring to the correct spot. If your heater doesn’t, most of them have a removable dial. You can take the dial off (without turning the knob) and place it back on so it’s pointing to the correct temperature.
- Repeat steps one through three to make sure it’s calibrated correctly. Once your heater temperature matches your thermometers, you’re all set.
If you’re having trouble calibrating your heater, or if you have a heater that’s not adjustable, you can override the internal thermostat with an external control.
When Can I Take My Heater Out?
Usually, ten minutes or more between when you turn your heater off and when you expose it to air is a safe bet. If your heater is set to around room temp and wasn’t running when you unplugged it, a minute or less is probably a safe bet.
If you expose your heater to drastic temperature swings your heater can crack, which is a particularly big concern if your heater is glass.
Should I Turn My Heater Up/Down In The Winter/Summer?
In general, no – with the exception of heaters that are always on (like cables.) Since your heater is set to a specific temperature, say 80, it will turn on when it gets below that and turn off and stay off when it’s above that.
With that said, it depends on your room temperature. If your tank is in a particularly hot place in the summer, it may be wise to turn it down slightly. The risk of overheating your tank lessens since it will have time to cool overnight. I’ve had to do this on a few occasions, just make sure you keep an eye on the highs and lows of the day, so your fish aren’t dealing with massive temperature swings.