Well if everything was 0 then it wasn't bad water quality. If you're cycling a tank then don't do 100% water changes, only do a ~50% when the ammonia or nitrites are >.25. I guess I should've asked if you're running a filter on the tank? If its unfiltered 2 weekly 50%+ water changes, one with a good vacuuming should've sufficed IMO. I would lessen the amount of water changes since he's in a 6g, part of the current problem could be that he's stressed because of daily water changes, some of them don't like it. I would do a 50% or so every 2-3 days & only replacing the Aq salt for the amount of water removed.