LOL, believe me that's one of the reasons why it's gotten so bad... it started out as a nice little patch on the driftwood, looking like a lush moss. Well I let it be since I liked it there, and that's when it started spreading to the leaves
If it only grew on the rocks and driftwood I would love it.
Before I knew exactly what it was I thought BGA was nice because it made the sand a more interesting color...
Actually it turned out OK because starting EI made it go away after ~2 weeks. As for your issue, I've seen enough duckweed to cover a 200 gallon tank go for $1, so if you can find someone wanting to get rid of it, you can probably add it faster than the goldfish eat it to experiment. I'm not sure how fast they'll eat it, so I don't know how sustainable that is. Excel melted my vals, but it took a couple weeks to really knock them back, and they are slowly recovering. Raising your light an inch or two, or putting a layer of fiberglass window screen under it would cut down on the intensity without removing an entire bulb.