Water gettin warm on a cold day?

its a cold raining day here but my tank water is 20 deg and climbing! is this happening to anyone else? ive been watching it going up a deg every hour.

How big is your tank? A smaller tank will heat up quicker than a larger tank. Even though it might be cold outside, what is the temp like inside? If you have the tank near a heater it will increase the water temp. 1 degree every hour is a lot of heat. To raise the temp of water by 1 degree requires a lot of energy. If you have a small tank and a light, it may be possible to heat the tank with the light (a small amount). I have a hood on my tank, I always leave the hood open to allow excess heat to escape. I also leave one of the glass lids on the tank open so it will also allow heat exchange and oxygen exchange with the surface of the water. Also, appliances can add heat to the tank (if I remember correctly). I read somewhere that have a canister filter with a lower power requirement can minimise the heat put into the tank. Also, what sort of thermometer do you have? I have 2 in my tank, a digital and a alcohol one. My digital one is funny sometimes because it will say the temp is for example 18 degrees and then i'll watch it and it will change to 20 degrees a short time later, its not possible to heat water that quick without a heat device. Maybe get another thermometer and get a 'second opinion' from it.

I hope that helps.

i have a 4 foot tank, which is quite large. not using a light, no heater in the room, but i was thinking that is could be the external filter, as it was quite warm. my thermometer is the type where u stick it on the glass, but i will purchase another one today.
fanx

3 of our tanks have filters - internal and external - and water temp in them is always warmer than the 2 without. So very prob filter warming it up. Water even feels warmer - current temp on 3 are 19-20deg. Other 2 tanks are about 16/17deg. (And weather is cool and drizzly here too!)