Anything that adds energy to the tank's water can end up raising the tank temperature. An oversized filter pump can be especially troublesome that way. The wattage of your filter pump is close to the heater wattage equivalent that it will provide in terms of heat. If your filter rating says it takes 25 watts to run, chances are it will heat your tank water as if you had added a 20 watt heater with no thermostat to shut it off. The 20 watts becomes heating power in your tank and a small enough tank will be affected by it. Most of us do not notice the effect because the tank has enough surface to lose most of the heat that a filter adds. The net effect is that the heater doesn't run quite as often, not a real rise in tank temperatures. I work in an industry where we actually intentionally use pump heating to boil water, so I am very familiar with the effect a pump can have on water temperature. In a small system with a large pump, I have seen temperatures move up at over 50F per hour. A typical aquarium pump does not use enough power to begin to approach that kind of numbers though.
