Our big tanks (4x2x21") 110s behave similarly in the Summer. They have temperature controllers on the heaters also linked to nightlights so we know when they are on and they are NEVER on in the Summer.
The tanks are lit by 3x54w T5HO and have sumps with Eheim pumps in them. The cold water out of our taps is about 80F in the Summer and the combination is enough so that they never cool down, they just stay about 80-83F all Summer. The tanks also have canopies which likely help keep them warm as well.
This is a pretty common occurrence, if you read in the marine/reef world they often have major heat issues - sometimes necessitating chillers as well as heaters. Fortunately our Africans don't really mind it a bit warmer so we just leave them be.
We did change our 55g to LED lighting in a tank without a hood and it is notable what a difference it makes. Not so much in temperature as this tank is smaller (and less thermally efficient) and never had a hood, but it used to have 2x39w T5s lighting it and now has LEDs sitting about 12" above the tank. What is interesting is that there is virtually no hard water staining on the tank lids now which I attribute to the lids being much cooler so there is basically no evaporation of water that deposits hard water stands on the lids. It was a very noticeable effect.
