I have the 65 on my 55 gallon tank and I couldn't imagine putting the 125 on it. It would be WAAAAY too big. Even the 65 is pretty big and probably pumps out 150-200 gph.
It is a great skimmer for the money. I empty the cup about every 2-3 days. It is always loaded with very dark liquid and nice chunks of dark green sludge.
Make sure that you read the setup instructions very well. The return needs about 1" clearance from the bottom of the bubble diffuser. You will have to trim it with scissors. If you don't, microbubbles will go into your tank and the skimmer will back up, overflow, and leak all over your floor.
Also make sure that you close the break-in valve about 50% of the way and open the water level dial 100% of the way. After about 3-7 days you can open the break-in valve 100%. Keep in mind that water-level dial lowers the water level in the skimmer the more you open it and raises the water level the more you close it.