Filtration for a 125 - how much turnover?

you should have a turnover rate of 8-11 times per hour. I would put a wet/dry trickle and a big cannister on it, like an eheim pro series or filstar xp3. Co2 will be harder to work with becasue you cant have to much agitation, and i dont know how those wet/dry filters are.

This isn't going to meet a lot of peoples recommended flow rates, but I think a pair of Fluval 404's will do the trick. $90 bucks a piece at Big Als...

I would never cheap out on filtration. Thats just my opinion on it but i would spend the most on that.

Two fluval 404s are not even close to proper filtration. Never go less than 8-11 times the tank capacity for an overturn rate. I spent more money on filtration for my 135g than I did on the tank!
you really need to set aside alot of funds just for filtration. I've got really limited funds and it took me 3 months just to get all the necessary equipment for my 135g.
I've got an aquaclear 500, rena cal xp3, an aquariums systems maxi jet mp900 on my 135g.

What cdawson said was very helpful. Except the AC will make the co2 gas off if you get it. So you could get three cannister also. I would get different brands just to mix it up a little imo. If it were me i would be the xp3 and the best pro series eheim. Then just a smaller cannister with like 200-250 gph.

It is were me, i would go with the wet dry and hte xp3.

I'm not sure where the 8 to 11 times volume figure is coming from. I've seen it and other figures from time to time (like 5 times an hour) but it seems to me… it really depends on what you're talking about.

What kind of filter? What kind of tank (planted or FO)? What kind of bioload? What kind of fish?

My Eheim 2213 is rated at 116 gph, turning my 30g over roughly 4 times an hour. Eheim recommends it for a 66g. They seem to think 1.75 times an hour is sufficient. They have a filter that would turn my tank over 8.8 times an hour… they recommend it for 250g tanks.

They recommend the 2217 for tanks up to about 160g. I'd get that.

I'd get the 2328 if I had more money and wanted to go in for the bells and whistles. (I'd add CO2 before the bells and whistles).

I wouldn't put an HOB on a planted tank or on a big tank.

Unless you're going marine or discus, eheim is kind of pointless because their canister filters are beyond expensive. I never bought a 2217 just because the price of it almost matched price of my tank. I spent 270 bones and I've got more of an overflow rate per hour than the 2217 alone. Remember carpguy chronieboy is on a budget and eheim will not help in that cause.

I would put 3 emperor 400s on it.