I do absolutely nothing for my ferns! No filter, no plant food, no special lighting. Just a desk lamp with an LED light, which shouldn't even work for plants! XD
I've used salts with no problem, and they are supposed to get big, but if you don't give them anything to help them grow the will grow super slowly.
Very hardy plants!
Proud owner of Victor, the Royal Blue VT!
Magic, the Black New Zealand Bunny!