What should I do now?

Looks like my angle fish is going to die as I was misinformed by the petshop. Right now he looks like he is struggling to stay afloat. :( I treated the water for chlorine used Tetra easy balance for the nitrates and have a heater in the tank and have been doing 50% water changes every other day. The only thing I think I did wrong was not let the tank go through a fish less cycle. Is there anything I can do for this fish? If it dies should I start over with a empty tank for a month?