I have a System six and two Explorers. The only problem I have had with the Six was my cats scratched it up pretty good by trying to bite the fish. One of these times she managed to actually crack a small section and over the course of about 7 months it became a slow leak but it was not too much to just order a replacement acrylic tank from Big Al's. The water also stays between 76 and 80 degrees so I use it for a Guppy tank and they breed like crazy in there. The explorers will also stay that temperature and they have not cracked on me. I find these perfect for bettas since you don't need to change the water out every other day, it stays in the good temperature range and you can put more decorations for the betta to play with since it's two gallons. I have never had a 3 or a 12, I would think the 3 gets pretty warm due to the light and small water volume and the opposite would be true for the 12. Overall though I would not recomend the Eclipse System tanks because you can get a better deal by going glass with a cheap hood and a better filter. A System 12 will cost about $80-90 but it cost me only $60 for a 15 gallon with a perfecto hood and Penguin 170 filter which is better than the filter that comes in the System 12.