I use a liquid fertiliser, just a general purpose one. But honestly if you are going low demand plants like anubias or java fern they probably dont need ferts as long as there are fish in there. I have kept java ferns without ferts, but definitely better growth with, although the constant trimming and removing of plantlets is a pain. I probably remove and throw away about 1/3 of the plant growth every 5 or 6 weeks to maintain them without it becoming too overgrown.
I keep saying this, but i think i could keep java fern alive in a bucket of water in the shed.
Put as many or as few plants in as you like. They dont need company. In fact, get one and you will be able to cultivate (is that the correct word?) more plants from that one in the future if you wish. All of that growth i mentioned as throwing away would be viable new plants.