well, duckweed will grow in just about any light ... similar to star-trek's tribble, the more (light) you feed them, the more they will reproduce!
anacharis will grow well in low-medium light you just need to wait for it to adapt., another is hornwort or foxtail, green cabomba
except for the duckweed, those are all bunch plants and inexpensive
problem being with ponds is unless a plant is nasty, the fish will most likely eat it. my parents goldfish can consume a whole bag (1 qt ziplock FULL) of duckweed within hours. The also enjoyed eating anacharis, amazon swords, arrow-head, pretty much anything ... the anacharis grew fast enough to combat their eating, but didn't look so good.