[Peter Finke]

In a normal aquarium tank filtration by growing plants is possible, in a blackwater tank the idea is nonsense. The best “filtration” is frequent water change. Small “filters” bring movement into the still water, they cannot “clean” the water. Especially for beginners they are useful for consolidation of the mind, nearly nothing else. I would always say people should use a small sponge filter, but the decisive thing is frequent water change.
I have bred all Paro species in small 10 liter-tanks without any filter and water movement, but frequent water change, twice a week is good. And floating Ceratopteris is good for consuming the small amounts of N that build up in the water from the little feeding of the fish.