Just lost my whole tank! need help and advise

I have a 55 gal tank that contained 2 blood parrot cichlids, 2 sevrum cichlids, some tiger barbs and skirted tetras. Nitrates and ammonia both tested at 0, PH 7.4, and water temp 78F. I tried to surprise my wife by sneaking some red tiger barbs into her tank. After watching the blood parrots actively hunt the new tiger barbs, we decided to remove the tiger barbs from the tank until they got a little bigger. About 2 days later we noticed first signs of ick on the larger of the sevrums. I started treating immediately with melafix.. After treating everyday for 7 days per the instructions on the bottle, the fish were looking worse by the day. I got panicky and went and bought some super ick cure. I did a 10% water change and treated with super ick cure, and added aquarium salt. After 48hrs, the fish health was still deteriorating, so I did a 25% water change, put the charcoal filters back in and let it run 1/2 the day, then treated again with super ick cure, 48 hrs later all the fish were dead.

It`s too late now because all the fish have died except for the red tiger barbs that I had just bought. I just want to know what I could have done differently. Did I over medicate? I assumed that the new fish brought in the disease, but the new fish are still doing well. Thank you