since hornwort floats it actually gets a lot of light even if your wpg
I wouldn't worry about fertilizer to start with, its just a quick way to provide a bit of a buffer for fish in the tank. Its cheap & it does suck up alot of nitrogenous wastes.
You have to be careful of introducing other things like snails though (a 5% bleach solution (1 part bleach:20 parts water) can help with thus. Look up bleaching aquarium plants for more info.