How can we suggest a treatment if we don't know what's wrong? There's little info to go by.
Melafix is useless normally, it treats gram positive minor bacterial infections. Most of fish diseases are caused by gram negative bacteria so this should be the last choice. You can try salt baths to relieve the fish but with the rate these are dying you have a stubborn disease going on and salt can't cure anything. Not all fish can tolerate salt too. Whether bacterial or parasitic it's impossible to tell from your description.