OK! I think I may have located the source of the problem! I did a full tank clean today, took out all the water, cleaned all the ornaments, the pebbles, everything. And I found...the catfish that went missing a few weeks ago Poor little fella! He was under one of the bigger ornaments, so I didn't see him when I was vacuuming the pebbles in the water changes.

Anyway, hopefully the water won't green up so much now!