I had 2 2217's on my 125, and it wasn't enough. I ended up buying a 2262. A 2217 is MAYBE enough for a 75 gallon, depending on stock. I think a 75 needs a little more than 1 2217, and a 125 needs a good bit more than 2 2217's.
I had two 125g tanks running the same setup which consisted of both with running (2) 2217 on each side and a single AC110 in the middle. That performed OK. I think running just one will be abysmal.