I ran straight bioballs in a fluval on my 35 corner with africans and never had a problem. Bioballs are great for wet drys due to the surface area for air and water to mix to all the bacteria grow really well. Well when it' submerged, you still have all that surface area that the bio balls provide, however the density of the bacteria ends up going down due to oxygen levels dropping from the wet dry. But it works. Or you could use coral. There's plenty of surface area for bacteria to grow on large coral substrate. All you're really doing is providing a place for benificial bacteria to cultivate. The fluval stuff was really porous that's why it worked well and it was inert as far as changing the chemistry of your water. if you've got south americans i wouldn't put coral in your filter. Just a thought though