I just called my AquaTek and he says middle of T-fitting is 12", bottom of T-fitting is 11.5" and sump waterlevel should not be higher than 11.5". In fact, his opinion is that the closer the sump waterlevel is to 11"-11.5, the more efficient this skimmer will work, probably run quieter as pump is fully submerged and less water return noise as the exhaust just gently falls back into the sump. HTH Christine