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If it's the same deal as the Aquaclear 20 (which I use on my 10g All-Glass QT), you have to break off the "leveler bar" which is at the base of the motor behind the intake tube. It's a little black plastic bar that sticks out... It was on step 5 of setup (Attaching Leveling Device) and I must admit, I thought I was breaking the filter when I did it, but then it fit perfectly!
Nope, not the leveler bar, it's the top of the filter between the return lip and the body that isn't wide enough. Most likely because it's not made for a tank that big? Fits fine on an old 20 gallon I've got. I was only putting it on the 125 gallon to get the bacteria colonized before starting up a 29 gallon, which is where it'll end up. I just took the bio-media from the AC and stuck it in the sump of the 125, hopefully that'll get the bacteria growing.
That should work fine. You can just leave it there for a week or so and it will be ready for your new tank. I'm sure you're way ahead of me here, but make sure you add fish or another source of ammonia at the same time you move the media to your 29 gallon so the bacteria will stay alive. Good luck!
20 20, i'm having the same problem putting a AC300 (70) in a 75g tank. the lip is too wide. so did you resolve the issue.
i've thought about gently filing the lip away so that it can fit.
Actually, it wasn't a real issue, because it wasn't intended to stay on the 125. I just wanted to run it on the 125 for a week or so so that the bacteria has a start when I put it on the 29 gallon I'm setting up. It fit's fine on the 29 gallon. I ended up taking the ceramic rings from the AC and put them in the sump of my 125 to get the bacteria growing on it.