I had greenwater for months in one of my tanks. Blackouts, water changes, clams, bamboo shrimp, no feedings, CO2, etc, all didn't help. I dosed hydrogen peroxide and it cured up my problem.
Add 2.5 ml of 3% hydrogen peroxide to your 5.5 gallon tank. I overdid the peroxide in my tank, and it nuked some plants, the snails and a few of the fish. You might be alright with such a low amount in your tank however, although it would help to add aeration until things settle, or to remove the fish.
My big tank that I just setup looks like it's going greenwater, if it does, I'm going to do the same thing.