If the mouth is rotting away and the fish are bleeding on their sides then it is probably mouth fungus (Columnaris). Normal fungus is white and fluffy and should respond to most medications, (Protozin, Myxazin, etc). Columnaris will need something a bit stronger, (usually tetracycline based medications available from the vet) and this will wipe out the filter bacteria. Unfortunately if you don't use a strong anti-biotic then you won't get rid of the disease.
When treating the tank you should reduce the feeding to once every couple of days and do daily water changes and gravel cleans to dilute any ammonia and remove the gunk that is produced by the fish.
To work out the volume of water in the tank
measure Length x Width x Height in cm
divide by 1000
equals volume in litres
When measuring the height, measure from the top of the gravel to the top of the water level. If you have big rocks or driftwood in the tank, remove them before measuring the height.
Before you treat the tank do a 50% water change and complete gravel clean. This will reduce the gunk in the tank and allow the medication to work more effectively. It will also lower the pathogen count in the water and mean there are less nasties around to infect the fish.
Remove carbon from the filter before treating otherwise it will absorb the medication out of the water. Increase aeration/ surface turbulence to maximise the oxygen levels in the water.