Apart from activated carbon/charcoal and sponge, what else is used in a external biological filter?

Question

Speaking about freshwater filters like Fluval 4 external etc.

Answer 1

Most of use three kinds of filtration:

1) Sponge -- This is the most important part of the filter IMO. The water running through it is cleared of debris and the sponge provides plenty of area for the good bacteria to grow and process out toxic nitrogen products while the water is running through. They should be taken out and given a quick swish and squeeze in UNchlorinated water every week or two and there is no need to replace until they get unusably ratty (and I've never had that happen yet.) Replacing them in fact gets rid of a major source of your good bacteria and can cause a dangerous spike in ammonia and nitrites in your tank.

2) Biological -- These are white ceramic rings or pellets. They simply offer more area for bacteria growth and aren't mandatory in the filter. Like the sponges, they only need to be rinsed once in a while, and not in untreated tap water because chlorine kills the bacteria.

3) Carbon -- This insert filters some chemicals out of the water. It removes medicines and water treatments so it needs to be taken out if you're dosing for illness or algae, etc. (Though chlorine neutralizers and the like work almost instantly and are done with their job by the time they get filtered out, so no worries.) You don't need this unless you have tannins or other contaminants in your tank that are discoloring your water for example. They go bad after a month of use and are not only ineffective after that but eventually start releasing dangerous toxins into your tank.

Many experienced hobbyists are running power filters with nothing in them but multiple sponges. Combined with regular water changes, it's really all you need.

Answer 2

something like bioballs which are supposed to increase the surface area for bacteria to grow on.

Answer 3

those weird biowheel things that are supposed to house bacteria that turn ammonias and nitrites into nitrates.

Answer 4

Depends on weather it's fresh or salt water !