Before you have a heart attack, no, I don't have fish in my tank yet! But I'm so frustrated 'cause I want those fishies now! My first heater I tried was an Aquarium Gardens non-submersible heater, and let's just say it enjoyed changing the temp. itself. As in, one day it was 85 F and the next day it was 75--and I didn't change the temp. The second one was an Aquarium Gardens submersible heater, which worked great--until I left it over night. The next day, the temp. had dropped, like, 15 degrees, and I couldn't get the thing to turn back on, no matter what I did. Needless to say, both of those are happily (or not so happily) back in Petco. Right now I am trying another Aquarium Gardens submersible heater, and it's keeping the water at a good, constant temp., but I had to turn it so far to the left just to get an acceptable temp. that I'm afraid I won't be able to raise the temp. if my fishies get sick. Does anyone have any advise on how to make the things work? Or should I just try a more expensive heater? Are there any brands that work really well? By the way, I'm aiming for a temp. of 80 F and 85 F if the fish get sick. I'd appreciate all the help I can get!