im not wanting to contradict anyone on here, but i have used formalin and pp to treat planted tanks.

What i do is treat with formalin at a rate of 2-2.5ml per 100 litres for 7 days depending on how sick the fish are. I then add pp after 7 days which actually bonds with the formalin to make it less toxic according to a pharmasist i have done some work with. PP is only active for about 45 mins and is then inefective and will change colour as it oxidises. I then use pure vitamin C tablets from a pharmacy or health food shop. The vitamin C must be 100%pure with no suger. I crush them up and add them to the tank. It absorbs the pp coulouring leaving the tank clear and also stimulates the fishes appetite