sweetreef... I just posted the following on Monday, so it should show you just how important maintaining a constant temperature is:
'I was away this past weekend, and upon returning yesterday, I noticed my pump was running very loud. I turned it off and put my hand in the sump to pull it out, and I noticed that the water felt very, very, warm. I looked at the temp and it was 92 deg. I immediately turned off my MH's, and gradually added cooler water to try to lower the temp in my tank. The temp started coming down and was around 82 deg by the end of the evening (78-79 deg is normal for my tank). I kept the PH's running all night for flow, but my sump pump was off. But, because of my stupidity, the temp this morning was 72 deg. I failed to realize that my heater was in the sump and wouldn't hold the temp in the tank with the pump off. I immediately noticed my CB and Hippo (and a Peppermint Shrimp) were dead; my Yellow Tang was lying on it's side (but doing better now); my male Perc is still not doing well (but fem seems fine); LB is ok; and GC's are fine. I have yet to spot my Dragonet and RG. (And I just noticed my CB Shrimp lost a front claw.)'
As you can see, the change in temp over a 24 hour period was the killer. Luckily, the remaining fish came around and are doing fine. Even if your tank maintains a constant temp w/out a heater, I think it's worth a few extra dollars to make absolutely sure it does by using a heater.