[mnemenoi]

Welcome and glad you found a great site. My recommendation is that goldfish can eat many plants, especially mosses and smaller types. Pellia and Fissidens both are easy to grow without any real need for a required substrate. Their only real need is a decent light for their tank. What sort of lighting/wattage do you have on there now? I would suggest something with a 6500K spectrum. With T5 fixtures a dual (2 bulb) will get you 1.29 watts per gallon, while a quad would achieve 2.59 watts per gallon. In the long run I imagine you would be happier with the quad as you'll be able to keep more in the future.