refugium and sumps

[GregJ]

I'm probably going to feel stupid once someone anwsers this for me, well here I go. My question is about how they work, flow wise. If you have an outtake going from your tank down to your sump, at which point you would have a pump connected to another hose going back up to the tank intake,how would you control the amount that is being pump up. If the pump is to fast, then won't their be to much water being pumped up to the tank, or do you just make a really big outtake hole? I wanted to try and make one using something like a small submersible pump or powerhead.

Thanks,Greg