just remove all bio filter, stone, plant.etc....completely empty the tank, fill in 100% new water and let all of the discus stay inside again...Do a prophylactic treatment with PP at 0.2gm per 100liter of water (one day bath) follow by 2nd, 3rd, 4th and 5th day change 100% water again…if not much improve try antibiotic for 3 days bath..
Don’t mix formalin with PP or even put near to each other...may explode..