The center drain which is a 3 inch PVC shower drain that has been inserted into and glued to a short section of PVC pipe. A piece of plastic was cut to make a shim that looks like a washer. It fits under the lip of the shower drain and sticks out a little. This is to keep the shower drain lip from pulling through the Uniseal (TM). It doesn't take much to keep the shower drain from slipping through, so any waterproof material should work. Perhaps a hardware store has a rubber washer that is right size? Or a piece of aluminum? I used what I had on hand which was a plastic wall covering.
A side view of the shower drain, plastic shim, Uniseal(TM) ans short section of PVC pipe.
To install the center drain you first must bevel the lip of the end of the PVC pipe with a grinder to make it easier for it to pass through the Uniseal(TM). Or you can cut the end of the PVC pipe diagonally but you'll have to cut that end off once you get it through. I prefer the former. You will also need Vaseline (TM) smeared on the PVC pipe to help it slide through easier.
Anyway, after gluing the PVC shower drain to the PVC pipe pop the Uniseal into the hole in the bottom of tank. Next slide the plastic washer shim up the PVC pipe under the lip of the shower drain. Then shove pipe though the Uniseal(TM) until the shower drain is level with the Uniseal(TM). Now you have a leak proof center drain where fish can't escape!
Another reason I prefer a Uniseal (TM) over a bulkhead fitting is there is less of a lip for solids to get trapped up against vs. a bulkhead fitting.
