I will comment but there are others more experienced.
1. I would return to daily WC for the amount of fish that you have, or at least 45% every other day. With 12 in a 55, I would be making plans for a larger tank as they grow - probably a 90gal minimal. Even then you are looking at daily WC.
2. If General Cure seemed to help, I don't think you did it long enough. I would go 10-14 days.