There is only one hole in the overflow box? Usually there are two. Most medium-sized tanks are drilled 1" for return, and 1 1/2" for drain. That's how my 90 is drilled. I wouldn't want a 1" drain on 125 gallon tank...that's too small. 1" will only handle 600 gph, and it won't take much to clog up. You should have room in the overflow to drill a second hole no?
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~ Mindy
SPS fanatic.
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