Are Ferns toxic to an tank?

Hi thanks for all the welcoms in the new member section.

Last night as i was busy planning my new planted tank i tought of somthing, what about a smaller shallow tank setup ontop of my tank and let water run through it with the roots of the ferns hang in the water acting as a filter of some sort. this tank will be fed with a small powerhead about 10-20l/h at the one end and back to the main tank on the other end.This tank will take up the whole back half of the main tank, But now a was wondering if fern might be toxic to the rest of the setup? Hope it make sence.

 

Depends on the fern, but I'm sure this won't be a problem. But be careful with whatever substrate, nutrients etc you put in the plant tank, as these will run into the main tank. We have a tank here at UCT where a delicious monster has its roots running into a fwater tank and its very happy for the constant supply. At the Cape Town aquarium the river ecosystem section has a moss and fern wall where water runs down into the tanks - looks fantastic!