But it is very easy with sponge filtersFor biological filtration, you basically need surface area for bacteria to colonize.
What is best can depend on cost or can depend on efficiency.
Personally I just go with ceramic rings, they aren't too big, nor too expensive.. in an average canister filter.
Sometimes I want to try foam, but never have. Maybe one day but I wouldn't rule it out.
At the end of the day, I am more worried about mechanucal filtration, if there is no detectable ammonia in an established aquarium I don't see any reason to worry myself about bacterial filtration any more. Having ceramic rings allows me to remove some from a filter to seed a new aquarium and fill the space left with more.. it's not so easy with blocks of foam.
If you want cheap, go to your local home improvement center and buy lava rock designed for a bbq grill.
Here is my aquarium with matten filter and sponge filter:
