http://freshaquarium.about.com/cs/tipsandtables/l/blheatersize.htm

Having said that, at a certain time in your hobby, it may be to your advantage to have a room set at a certain temperature for your fish. The enclosed (water-spill-proof) heater I have in the fishroom is a lot cheaper to purchase ($200 vs $18 times 70 aquariums), far longer lasting than most aquarium heaters and cheaper (less expensive?) to run than having a heater in each aquarium. (And small tanks are a bear to heat otherwise.) Lastly, the static sounds on radios caused by corroding sensor points in the aquarium heaters are avoided.

A corollary to that is to keep fish whose optimum temperatures are also comfortable for you. One winter day I was invited over to the fish house of a cichlid guy. When I blundered in, out of the February weather, into the 70 x 15 foot/21.4 x 4.6 meter enclosure, the 84 degrees F/ 29 C almost caused me to pass out!

I much prefer to reside in the middle/ lower 70s F/ 20-25 C. I have a three tiered stand of ten gallon tanks. Cooler water fishes can go on the bottom. Guppy types go on the top where it is 4-6 degrees F/ almost 2C warmer.

That summary of Stan Shubel's study on temperature and guppy life expectancy is below. Higher temperatures will significantly lower life expectancies. And the guppies are more expensive to keep warm.

http://www.guppylog.com/story/2005/4/20/19226/2648