I will buck the system. As long as the temp of the house doesn't drop below 70 degrees (some might even say lower than that

) you don't need a heater.
Burn me now, but you don't need them.
I have mbuna tanks that breed on a regular basis and show bright coloration. All are kept at room temp (70-72 in the winter/76-78 Summer). This mbuna need 80 degree F temp water is a falsehood. Yes they are less active, less agressive. I feed them less so they don't grow as fast, but they live just as long if not longer, the breed like rabits, and the coloration is outstanding. I have done both ways, room temp is the cheaper way (capital cost of heater purchase avoided and electrical cost per month reduced)
Ok, I am braced and ready for the bash...
(also, mbuna are not herbivors if they were you could let holding females release in a tank and they would all survive, you can mix feeding pelets and flake in the same feeding without getting bloat, you only need to dose a tank for the water changed even if you add water directly from the sink, you do not need 10x water movement for filtration, and more complex = not better! Sponge filters all the way baby!)