You should be able to use a check valve on the fill line in order to stop the water from flowing back out.
Once you remove the hard tanks check and see how much space you have. From what I remember you should be able to use the Fly High 750lb. Fat Sacs (50" x 20" x 20") back there to optimize your space.
Sounds like you have an aerator pump system. If the drain pump that is connected to the hard tank has a 3/4" threaded inlet you might be able to use the 3/4" Flow-Rite > Attwood Tsunami 800 Pump Adapter
. We can get you taken care of what ever the thread pattern may be.