Heater not heating high enough- is this okay?

SuspiciouslyFishy

In my bigger tank (just under 15 gallons) the heater is only heating the water to 20/21 degrees Celsuis, despite previously happily heating to 25, which it's set to. I don't know what kind of heater it is, but it came with the tank in question, so must be a couple of years old, and has been running for about a year of that time. My fish and snails (Otocinclus, Neon tetras and ramshorn snails) seem happy and are swimming and acting normally, with brilliant colour, despite 21 degrees not really being in their ideal range. The tank has a lid which is on at all times, with an extra helping of clingfilm to stop the otos jumping out. The light indicating the heater is on seems to be on all the time, as far as I can tell.

My ambient room temperature sits between 14 and 16C for most of the day, as it's winter. I wrap up warm, and heating is expensive. When I feel sorry for myself I put it on for an hour or two to heat up to between 18 and 22C, which is where it would be at most of the time when it's not winter. My other tank is heating to the temperature it's set to, but admittedly, that tank is only 7.5 gallons, which can be a big difference for heating water.

So, is this a sign that I need a new heater? Should I leave it alone as everyone seems happy? Is it just because it's winter and the air is colder?
 

Prestoncichlids2029

I think you should swap your heaters in the tanks to allow the bigger one to have the better heater even if it isnt as many watts. 20/21 degrees Celcuis is a little low for those fish. (I’m gonna convert to farenheight just for the purpose of explanation, as I am in USA)…21 Celsius = 69.8 farenheight…an ideal temperature for those fish is around 75 degrees farenheight. This is okay, as it won’t harm the fish, but it only slows them down, they’ll be less active at a lower temperature. But if you swap the heaters, your other smaller tank may be heated fully with you bigger tank’s heater. Despite all of this, a 100 watt heater is all you need for your 15 gallon tank, I’m not sure what your financial situation is, but generally those can be found for around 10 bucks (USA dollars)
 

RayClem

The thermostats in aquarium heaters are not always calibrated properly. If your heater has a preset thermostat, there is little you can do except replace the heater. If the heater has an adjustable thermostat, try turning the set point up one degree C at a time and check the tank temperature after a few hours. If the thermostat indicates 30 degrees C, but the tank temperature is 25 degrees C, that is fine. I never trust the thermostat readings on my heaters; I always set them to whatever I need to maintain the proper temperature in the tank as measured by a thermometer I know to be accurate. Note: not all thermometers are properly calibrated, either.
 

KingOscar

I would not leave them at the lower temps for long, it could affect their health adversely.
 

Arvil

Your post did not indicate the heater size. Perhaps it’s giving all it can with the low ambient temperature. Consider adding a second heater to help maintain temperature. That way if one stops working, the second one will help. But, my previous heater began to fail, wasn’t able to maintain temperature even if it was up all the way, just couldn’t produce heat. Maybe that’s the problem.
 

SuspiciouslyFishy

I took a second look at the heater and I think I've found the problem. It's only 25 watts.

And a close look at the heater in the smaller tank reveals that one is 55 watts. I may have accidentally swapped them when I moved in May last year. If I swap them, the issue may well resolve itself.

Also, for clarification, the smaller tank is doing just fine. That heater is set to and maintaining a temp of 77F/25C
 

RayClem

The typical recommendation for heater size are based on an expected temperature rise of about 10 degrees C. However, if your ambient temperature drops down to 14 degrees C in winter, you may need a slightly larger heater to bring the tank up to 25 degrees C.

While a 55 watt heater might be adequate for a 7.5 gallon tank, you are probably going to need at least 100 watts for the 15 gallon tank. Using a bigger heater will not increase your electrical costs as the heater will only run long enough to bring the temperature up to setpoint. An undersized heater is going to run constantly in an attempt to bring the tank up to temperature. A properly sized heater will turn on and off as needed to hold that temperature.
 

TClare

I was going to suggest turning the heater dial up as the temperature reached often (always??) does not match what is says on the dial. However, as you say the light is on nearly all the time this suggests that the heater you have is not capable of maintaining the correct temperature, seems like it’s time to get a new one, as others have said 100W should be fine. I agree the temperature at the moment is too low long term for the fishes you have.
 

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