Do as you have been doing clean as much off by hand as possible and remove leaves that are severely affected. Doing a 50% water change weekly for a while will also help.
The lowest light fast grower I know is Hornwort, but with a 20G long you can also try Anacharis & wisteria. You will need to keep these pruned back since they will grow tall.
You may also want to ease up on the ferts or stop completely excluding the Excel until the algae clears up (about two weeks). |