I think I would lose the cartridge filter. It's not going to do anything for you biologically and it's going to clog really fast once you stock a bunch of fish. On my pool/pond I used a sand filter with a layer of gravel so that it wouldn't get clogged too fast. On your's I would probably consider getting rid of the pool filtration entirely and build a shallow bog type pond for lots of plants running the length of the pool. You could just have the skimmer drain into the bog on one end and use a pump on the other end of the bog/pond to return water to the pool.