Originally Posted by Byron
Heaters frequently maintain a temperature higher than the temp setting. The main thing is that the heater is maintaining a stable and constant temperature. You merely lower the setting (in this case) until the tank temperature is where you want it. For example, in one tank I have an Eheim Jager heater which are high quality, and to keep the tank at 77F I have it set to 73F.
Also make sure you have a reliable thermometer. These are sometimes not accurate, especially the digital stick-on type. The floating stick type tend to be accurate; it is wise to have two or more, and at first use both of them in the same tank to ensure both are OK.
The ambient temperature in the room can impact heaters too. Most manufacturers will tell you that the room temperature should be no more than maybe 10 degrees below where you want the tank temperature. If the room is colder, the heater will have to work harder to maintain the tank temperature, and may not be able to.
I have 2 thermometers. A floating type and a LCD stick on type, so ahead of you there. My heater has been unpredictable, either never getting to the target temperature, or going over it a bit. I can never get it right on target...
I did an experiment over night. I put the heater in the main tank compartment and low and behold, the temperature is right where I want it!
The water in the back filter/pump/heater/refugium compartment is a different temperature than the main compartment. Also the pump seems to be stronger than it needs to be and that part of the tank is lower than the main part. All this means is the tank could use a better design - more water flow into the back compartment to keep up with the pump.
Only problem now is my heater has to be 2-3 inches above the water and can't be fully submersed. This prevent me from putting my lid on. I'm going to buy a different fluval heater. The Fluval E Electronic Heater looks like a good choice.