Both frogbit and duckweed are classed as weeds by most places. In the meantime, I have both and am finding the frogbit a little harder to grow than the duckweed. I am still trying to get it to flourish in my planted tanks even though many of those tanks grow other plants quite nicely. As far as not being able to get rid of duckweed, I find that I have trouble growing enough in my other tanks to feed my rainbow cichlids in their tank. I am sure that goldfish people have the same trouble growing enough.