Can anyone tell me the difference btw Eheim's Professional and Classic canister filters? What's the difference between Pro I and Pro II filters? Is Pro II the newer better version? Would a Pro 2026 (250 gph) along with an eclipse 250 gph be overkill for a 37 gallon tank? Or should I go with Pro 2222 (132 gph)? Or Classic 2215 (164gph)?

I just want to do what's best for my goldfish!!

I can't answer all of your questions but since I just got an Eheim 2215, I was doing research myself recently. As far as I can tell, the main difference between the Pro series and the classic is price. The price difference seems to be accounted for by additional features in the Pro series like automatic priming, water outflow monitor, and such - the big difference seems to be in power - the Pro series of same or similar gph has a more powerful pump for out-put/spray. As far as I can tell, those are the basic differences. I don't really know for difference between Pro I and Pro II series.

As far as goldfish are concerned, I think there might be no such thing as filtration overkil (I wondered the same thing) - I woud think in terms of bang for the buck and usefulness. You will have an Eheim for a very long time as they are very dependable - so which filter will accomodate your long-range plans the best? For example, I bought one that is a littlr large for my tank now but will be big enough for my next upgrade - 30g. After that (moving up to a 55), I would add a second filter of equal strength to have one one either side of the tank. Then, I will get another Eheim 2215 because that model will always be available (and therefore, its parts, too). And on like that. So, I picked the biggest I could afford at the moment that had the most versatility for changes I would like to make later.

I can tell you that the Eheim runs like a dream, easy to clean, change, and even set-up (only difficulty I had was not all the parts were labeled and my lack of familiarity with canisters left me a little confused). The double isolating valves are great and make the whole thing relly worth the cost! THe filter is completely silent!

I have a 2213 for a small tank with 2 goldfish. They love the clear and fresh water so I think you can not overfilter youre tank.

If I'm starting brand new, I would go for the Pro III. I have 2 x 2213 and if I'm going to get another filter it will be another 2213 so that I don't have to stock so many different types of supplies.