Ok! So I've been away for awhile but I know have a 3 gallon Kritter Keeper which is filtered with a tetra 3i whisper filter and heated (I keep a close eye on the temp and it is very stable.) He's been in this tank for about a year now and has really thrived in it. I've had Cadence for about two years now and he is as happy as a clam. I know it's by a window, but I have a thermometer on it and the temperature is constant, and I keep a folder behind it to keep the sun from heating it up on the really hot days.
(also i saw something about not keeping betta's in kk. I really don't see any problems as long as it is heated, filtered, and large enough??? At least my kk was made for fish, idk.)