When you get it there will be directions on the back. You use it to keep the fungus away from the eggs I think it was somthing 1 teaspoon for every 5 gallons? I am noot sure but when you get it there will be instructions.
Be aware that meth blue will stain silicone, air lines, and any other softer plastic a nice sky blue. This is not something to be used in a show tank, unless you don't mind the blue tint.