Originally Posted by grumpygils
I put mine lower than any possible overflows when the pump is off. If you have a protein skimmer, they water height will affect that too. Mine is about 2/3 full.
Mc
ditto.. you want to leave enough space on the top of the sump such that if you're pump stops working, the amount of water that may be back-siphoned from the main tank to the sump will fit.
if this is the first time the sump pump is being turned on, you'll need probably a few inches of water above the pump, cause some of the water will always be in the piping.