your absolutely correct fresh water only. People have tried to put something similar for Saltwater tanks and there are a few but not much info just show off video. It's basically creating a self-sustaining eco-system in your tank where the entire life-cycle sustains everything other than having to feed the fish. This is the reason I mentioned have a large CUC because in freshwater you really depend on Nirite Snails, Shrimp, and some others to really be the cleanup crew. I never ever in all the years I had the tank ever had to clean the glass as the CUC kept it spotless, same for water parameters after about 6-8 months I just stopped checking it because it just stabilized itself really. Never really had fish die on me except at the beginning of course, shrimp bred like crazy etc.. I'll try to post a pic to show how healthy it was and how crazy my plant growth was....and yes understand that Coral are animals not plants and it's a whole new game, but I'm up for the biology challengeEDIT - PS - I googled the walstad method - it is designed for Freshwater planted tanks - not reef tanks - unless I'm not finding it.