It's going into an eheim 1250 pump. The specs I have says it takes a 1/2" male, but i bought that and it's too big. I'm assuming its 3/8". I want to connect it to my 1/2" return line.
I wouldn't chase the 3/8 fitting, I believe the Eheim pumps have metric fittings. Can you try using a piece ot vinyl tubing to connect between the pump and standard fittings? The vinyl tuning should be able to stretch enough to make it work...