If all of the fish in your tank are gilling on the bottom, it may indicate a depletion of Oxygen to the point that the fish can not function and will soon die if the is not cured or from ammonia or nitrite poisoning.
If the problem is with only one fish, at this point, no matter what the cause or how intensive the treatment, the fish will probably not survive. Most aquarists prefer to put the critter out of its misery, rather than watch it suffer until it finally dies, which may take several hours or even days.

