Join Date: Jul 2010
Location: Outside Philadelphia, PA
You can build a sealed sump, but consider that if you do this to maintain CO2, you also risk not getting the gas exchange you want to get new O2 into the tank.
Since your dealing with a larger tank, your likely going to want to use pressurized CO2 anyway. If you do that, you can crank up the CO2 to make up for any loss through the sump.
If you must have a systems with a sealed sump, I think you'd be better off using canister filters, rather than making this a DIY project. You also will not need overflows since the canister filters are sealed.