I hope these might help.....1) drill a hole on the return-pipe,where the water drains into your main tank(or you already has
one that stops the back siphon)
2) connect a tubing so that now it drains into your frag tank(make sure that the outlet of this
tubing cannot be submerged or else,there be a back siphon.
3) your frag tank has to be slightly lower than your main tank.
Happy reefing.