This could be the white spot disease. Its a diesease which causes white spot to appear all over the fish. the white spots contain mature protozoan parasites. the spts begin to rupture releasing parasites into the water. The parasites then form cysts. The cysts sink and attach to the aquarium floor. Each cyst can produce up to 1,000 tomites which are the free-swimming form of the parasite. The cysts then break open releasing the tomites into the water. Tomites in the water attack other fish and then repeat the cycle all over again. This diease spreads fast and thousands more can be produced in a single cycle. Yet you may not know that this is going on in your tank. My blue gourami has 3 big white spots which have been there for months and that is just the pattern of the fish. So it could just be normal for your fish to have the spot on it. Im guessing that it is not the white spot diease but if it is i dont know of any cures. So dont worry if the spots have been there for 1 week or so and know other fish are coated in the spots.