@MaloK Sounds like I have my tank upside down then! I planted the vallis and elodea in the sand as they have roots and I assumed needed to be anchored securely in the sand. I put the anubis up near the top, a I wanted some plants higher up, and they easily attached to the wood.
I do add duckweed, but the problem is that I can't grow it fast enough to keep up with them eating it. Although you can't see it in the pic, I have a tub of it sitting on top of the tank, but it doesn't grow very quickly. I have a trough outside with more in, but again, it only grows very slowly.
As for the moss balls, I have no idea if they are real or not, but the goldfish seem to ignore them. Not seen any evidence of damage, other than one which I pushed into a hole in one of the pieces of wood ('cos it looked nice there!), which got knocked out of its hole. They all sit around on the sand collecting bits of sand that the fish kick up.
@Colin_T I'll have to wait a few days until it grows back, but I doubt I'd get a clear picture. However, I think I know what you're getting at. In previous tanks (years ago) I had that slimy dark green stuff that comes off in a film. This isn't that. This is light green, and when I clean the glass, it comes off as a fine dust that float around until the filter sucks them up or I suck them out when doing a water change.
As I said, I did try using Tropica Premium Nutrition, but I didn't see any difference. Maybe I'll try again.
Thanks again to both of you. Any more advice very welcome.