Any small filter should be fine in the tank. As for lights, I haven't seen a kritter keeper up close before, so I'm not sure if there's any way to affix lights, how about a desk lamp over the tank? The heater mentioned should be ok, just ensure it's not too powerful being designed for a larger aquarium, what watt is it?
Those tanks hold roughly 3 gallons, right? IMO, I wouldn't keep bettas in anything under 5 and my betta lives happily in a 7.4, but that's down to personal preference I suppose. Most people on here seem too keep them well enough in 3 and smaller, so I suppose they prove it can be done. As a personal preference if going for a kritter keeper tank, I'd go for the extra large which holds 5 gallons. If you provide the fish with plenty of plants and stimulation, I suppose a 3 gallon could work.
Last edited by BettaBuddy101; 11-19-2011 at 06:39 AM.