I have a 900g tank in the barn. Two 100g rubbermaids filter it. Here is how I've done it.
A 3" suction bottom drain comes through the side of the tank and is valved before entering the bottom side of the lower 100g tank. The pickup is a 4" to 3" reducer coupling in the center of the fish tank and the flow out of the tank is by gravity.
A grate sits atop the first ledge in the stock tank and a 2" mat lays on it which has been cut to fit snugly against the sides. Other mats lay on top of the first one, filling most of the rest of this tank.
A mag drive 7 pumps water from the just below the surface of the bottom tank up to the top tank. This tank is grated at the first ledge also. Below the grate are 8 airstones. Resting on the grate is Bacteria House. Floating at the top are bio balls. I store all spare media here also, trying to keep everything well aerated.
Another mag drive 7 draws from the skimmer and pumps into the top of the 6" pipe filled with lava rock. That flow falls into the top tank also. A vertical 3" slotted pipe allows surface water to exit and elbows out the bottom where water return causes circular flow in the fish tank.
The bottom filter tank has double 2" shower drains through its bottom going to a single valved drain line so I can turn off the pump, shut the valve from the fish tank and drain this tank completely and separately from the others. The flow pumped from skimmer to shower keeps running while I do this. Open the valve once or twice to flush the pipe and the bottom and its clean. When I had almost 200 fish in hereI had to pull all the pads and hose them down every week, but with just the few small fish in there now little crud has built up and cleaning has been fast and easy. There used to be a mag drive 12 running this part of the system. It caused more problems cleaning the mats. The 7 pulls way less crud into the mats.
I have had the top filter apart and nothing has settled there except for BH particles. The $tuff does fall apart a little.
When I switched from the mag 12 to the mag 7 I had to move the suction for the bottom drain out to the center of the tank. With the 12 's greater flow stuff rolled to the tank perimeter and got picked up fine by the drain there. With the drain in the center, to help keep things clean, I added a couple airstones to the bottom drain and the bottom stays clear of anything.
So, filtering well with rubbermaid stock tanks can be done. Just don't call mine a Skippy.