I have a nano-tank for my 2 Sulawesi Elephants (and their little baby), and i have a internal filter, that has (in this order) : Course foam, Fine Foam, a few JBL Micromec balls (for biological filtration), and a little 2x2 cm perlon pad (JBL Symec Micro)
It works great, the NH4 and NO2 stay very low, almost gone.