Do you rinse and thaw your frozen foods in a fine net before feeding? I was told it's a good idea. I just do it under cold water from the kitchen sink tap then wring it out and feed in small portions. I feed once a day, I think it's good to keep them a little hungry with tails up and heads down looking for food just like in the wild. I also feed pellets and nori for the tangs and stay clear of flake food. Keep doing 25% water changes once a week if possible and your levels will get better but as mentioned it will take some time.