You can get a 8 foot diameter round PVC stocktank from Tractor Supply Company stores for about $200. Get a hole saw blade, a drill and you need a 2" or whatever bulkhead from a plumber supply store or AquaticEcosystems.com. Saw the appropriate sized hole in the center bottom of the tank, screw in the bulhead, it's gaskets will prevent leaks, and glue in a PVC drain pipe schedule 40 PVC L. Place your stock tank with bricks around the bottom edge to raise it slightly so it will drain to the middle. Run the drain pipe outside or to a sewer. Thats how I did it. Not bad for a girl, huh?
Large size rectangular mortor mixing pans can be gotten for $13 at Home Despot. They fit indoor enclosures very well. When you want to drain it, drop in a little pond pump from the garden dept attached to a garden hose and run it out the window. Bubye poopy water.