Methylene Blue is available in different concentrations. You need to check the directions on the pack to see how much you use.
You don't have to break down the fish's tank. Just move it into a spare container that you don't mind becoming blue and leave it in the water with Methylene Blue for a week. Drain and refill the main tank and in a week move the fish back into it. The wound should have healed in that time and that should be the end of the problem.
To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "How To Tips" at the top of this page that will let you convert litres to gallons if you need it.
Remove carbon from the filter before treating or it will absorb the medication and stop it working.
Increase surface turbulence/ aeration when using medications because they reduce the dissolved oxygen in the water.