Strange heat problem

So I have my 29g biocube with 16# live rock happily cycling (pictures soon)

I've got a heater, but it's not on. The room temperature is 70-72 degrees, and I'm shooting for 74 degree water. Despite everything, the water is about 78 degrees. Any ideas?

 

If you're going for a reef tank, 78 is well within the acceptable range.

The extra heat could be heat from pumps, the lights, the heater being off with its temperature dial, or any combination of those.

I would turn off the heater, and see how much it drops, to confirm or eliminate that possibility.

If that's not it, I would check the lights and the pump the same way, individually.

 

How many watts is the heater? If you have an excessive heater then it could potentially be heating higher than what you specify, I've had that happen on a few smaller tanks.