I have a heater in my 3 gallon critter keeper with my betta (not sure the wattage, but its Tetra brand and says its good for 3-10 gallons, so I think its a 25 watt?). Its non-adjustable, but says that it's supposed to keep the water at 78F. My problem is, its not doing it well enough. It drops to 76 occasionally, which isn't horrible, but its not ideal. Not to mention I don't have AC in my apartment, so the temperature of the water gets up to 84F on some days. With such instability, it can't be good for my betta (especially since I think he might have velvet + constipation. If anyone can help me with my betta, please visit my thread asking for help Here). I'm curious if anyone has tips for keeping the water temp any more stable than I can manage it now? I try to keep my apartment as cool as I can, but that doesn't stop it from getting up to the mid 80's. My heater keeps it at least at 76, and I don't quite have the money yet to try and find/purchase an adjustable heater that's small enough for my 3 gallon. And, I don't think having 84+ degree water is good for my fish, unless he's ill and needs water that hot.