I was given a 20 litre hex tank with a built in light, i purchased a 50w heater and a small filter. i filled it up turned the filter and heater on and realised after thirty minutes the temp got to 80.
I unplugged the heater, left the light off and 24 hours later still the same temperature. I have taken out the thermometer and it works fine in any other tank, as soon as i put it back in it goes back up to 80.
The tank is sitting in our kitchen a bright room but out of sunlight and any overhead lights.
I unplugged the heater overnight and the temperature has not changed. The thermometer works fine as i have tested it. Could it be the tank is just very good at retaining the heat?
well it depends on your room temp as well. I would think that the temp should've dropped over 24hours, but water is slow to cool. I guess it is possible that your tank is just better at holding heat than most. (BTW is your tank near any heat source, heater or stove-- this could be the prob.)
It is sitting on a worktop, with a fridge and a freezer below it. The temp is now 78, so it has dropped 2 degrees in 20 hours. At least i dont need to put the heater on at the moment.
I bet if you feel the undersurface of the counter you have the tank setting on, it will be warm from the refrig/freezer setting under! They do put off heat. So it wouldn't be a problem if you have your heater in the tank set at a minimum of what you want your tank to run, then if it cools because the other appliances are not running, the heater will kick in.
I bet if you feel the undersurface of the counter you have the tank setting on, it will be warm from the refrig/freezer setting under! They do put off heat. So it wouldn't be a problem if you have your heater in the tank set at a minimum of what you want your tank to run, then if it cools because the other appliances are not running, the heater will kick in. 
