Does anyone know if the marineland penguin 350 bio wheels can fit on the rim of a 125 gallon 6x18 tank?
Depends if you are talking about a glass tank or acrylic. Even the vendors for acrylic does different things for the top cover, so it will vary which ones you can put it on and which ones you cannot. It should fit a glass tank with no issue.
Yes, I have a 350 on an Aqueon 125g, and another on a SeaClear acrylic. I was able to drop them in place, with no modifications. Obviously, much additional filtration will be needed (depending on stocking and conditions).
I would buy an AC 110 instead, or two. I love both the 350 and 110s, but between the two... 110 all the way. I have several of both.