This is an idea one “Gardenman” on the Aquarium Co-op Forum came up with. He put the plates and lifts of two undergravel filters into two aquariums. He then covered the plates with two inches (5 cm) of 40 ppi charcoal colored foam. He put an air stone in one of the lifts and a powerhead on the other lift tube. The result were two surprisingly good-looking aquariums.
This will do a great job of biofiltration as the surface area is huge. It is also quite easy and surprisingly cheap to make. It is even cheaper than using gravel, given the current high price of aquarium gravel. This is simply a great idea! This is one of the tanks he did this to:
The plants grow quite well in the foam, putting roots directly into the foam. Note he has two sponge filters in the tank in order to “seed” the bottom foam. He removed the sponge filters after six weeks.
If I were doing this, I would probably use a 30 or 20 ppi Poret foam, but that is hardly a requirement. Because of the large top surface of the foam exposed the flow at any given point is relatively small, which means there is no force drawing the feces into the foam and plugging it up. And I would go with one inch foam.
Some cautions about alternative designs. Some want to add a sizable layer of gravel over the foam. This will compress the foam and make it useless. Others want to add a quarter inch thickness of foam under the plastic plenums. Anything, and I mean ANYTHING, which is placed under the plenums will prevent uniform flow over the surface of any undergravel filter. So never put anything under a plastic plenum.
Some will question why the foam won’t just plug up with brown gunk and stop the flow of water. The answer is that normally in 99% of all aquarium the conditions are such that the decomposition rate of the “brown gunk” is equal to the addition rate.
Now ANY filter can be overloaded. If one has some combination of VERY heavy stocking, heavy feeding, low protein food, poor aeration and/or no good inoculate then one may VERY RARELY plug up a foam undergravel filter. But this is not something for most hobbyists to worry about.