My pond is full sun, 8 goldfish (no koi), no soil or gravel. It is a 300 gallon plastic stock tank with straight sides (no ledges). I do have some concrete blocks in the bottom for the fish to hide. I have zone 5 winters, sometimes it freezes over the surface for a few days at a time. I would like to grow some things that will over winter w/o much fuss and come back strongly each spring. I don't care if any of the plants bloom or not, I just need green, something to use the nutrients to my algae isn't too bad. Any ideas?

Parrots feather should work.

As long as it doesn't freeze to the bottom, you should be able to get a small hardy water lily in there (in a container). You could likely put marginals in containers on top of those bricks, too.