Filter Heating My Water?


So I made a thread yesterday about my water being at a constant 82 degrees despite the fact that I didn't have a heater hooked up to it for almost 24 hours. Today I did a 75% water change with cooler water (about 68 degrees out of my tap) and still haven't plugged in a heater. Since the water change the temp has continued to rise and as I write this it's at around 78-79 degrees again. I used my laser thermometer on my fluval 406 filter and it's reading about 84 degrees where the propeller is. I just set this filter up on Wednesday. Did I put it together wrong? Seems awfully warm for just filtering room temperature water. The filter is located in a cabinet underneath my tank and the room temperature is around 73-78 degrees constantly.


The filter will heat the water a little but not that much. Is the tank by a window, heater, fireplace, cooker, etc.?
  • Moderator


Could be a combination of a few things.

You're in California; looking at the weather for Anaheim, you're up around 85F outside ambient. If you don't have air con, the ambient heat will heat the water. You say ambient is around 78, so that's your "room temperature", and everything will heat from there.

Lighting; can heat the water; depends on the distance from light to water surface, but not uncommon for lights to heat water by quite a few degrees depending on the tanks size.

Filter: They cool themselves through heat transfer, and it's kind of a vicious cycle. This is especially common in Saltwater tanks, where a lot of equipment is used. But in a nutshell, the filter keeps transferring heat to the water.

As an example, I had my Saltwater tank go from 78F to 90F just through ambient temp rising and all the equipment trying to cool itself, and the lights heating the surface.

You can put the heater in, as long as you know it's set properly. The heater only comes on when it gets too cold, if the water is too warm, it won't come on. I run my heater 24 x 7 in summer, and I get ambient temps around 86F+

If it's not sustained, you can just run an air pump to provide oxygen (higher temps = less oxygen in the water), if it's sustained, you may want to consider a chiller (but that comes at a cost, both $ and noise), or running air conditioning to bring the air temperature down. Running fans can also help, especially if the filter is in a cabinet, as the heat can build up in a cabinet.


Is the tank small? Does it have an enclosed hood or lid? That can also make the water heat up or stay fairly constant.


Most of my tanks are unheated, so I can compare air and water temperatures. A filter will raise the heat at most one degree celsius, but rarely even that much.
  • Thread Starter


Thanks everyone. The aquarium is a 40 gallon with a hood. I haven't been running the light at all but I guess this is just going to be my water temp with the weather outside being warmer. Woke up this morning and it's still at about 78 degrees in the tank so at least it's not going back up to 82


I've had the same problem running powerhead sponge filters in my tank. Didn't even need a heater...only a WC would cool the tank down.

I finally switched from powerheads to canister and the problem was solved.


It's probably your canister. A poor man's way of cooling the tank in clip on fans, one in the stand pointed at the filter, and one on top of the tank pointed at the water. No glass top. I have a 75 gallon with 2 canisters that runs around 78 degrees just from the canisters, I needed 72 to 73 degrees just for my shrimp so I put a fan in the tank stand, and one on top aiming at the water, and got a constant 73 degrees. You get a lot of evaporation but it's much cheaper than a $300 chiller.

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