One thing with heating and cooling I've found, is find the range of your tank without heaters or chillers as it changes with lights, house temp etc. I have a small two bedroom house to myself, and the tank heats up when I have parties because the door is always open etc. And then work with those numbers. I prefer to be cautious on the heating side, because if it malfunctions, I'd like the most amount of time to notice before everything gets nuked. OF course this must be balanced, by ability to heat the tank and not cause harm to inhabitants. On my 90, I use two 250's.