PondCare Aquatic plant food tablets safe for aquarium?

These tabs are meant to go in the substrate, so as to be taken up by the roots. Ferns and mosses don't have roots. These tabs are going to be useless in your particular tank, and will likely be far too concentrated for such a small volume of water, meaning you'll start seeing mive algae blooms feeding on the excess nutrients. I'd suggest you take out the one you already added, and stop adding any more. Maybe give them to a friend with a pond.