I have been doing biocubes, big reef tank, and now pico tank for about 4 or 5 years now... and I never stop to ask if you really need a heater. I was always told I need one so I always purchased one, but we had two heater incidents recently - one was stuck on and killed 5 cichlids in a freshwater tank and the other heater cracked and killed 3 saltwater fish in a biocube.... so i started thinking if you really need one?
I looked online at liveaquaria.com, and all the inverts, coral and fish said they required temperatures from 72 deg to 78 deg... I am run a 5 gal pico (cycling right now) without a heater and the temperature is 74 deg over night to 75 in the day as the ambient temperature stays the same... do you really need a heater? maybe in winter? but anything really require a temperature of 79 deg or 80 deg?