I do not like using formalin inside homes, it off-gasses and is considered a carcinogen.
The daily dose for formalin is 25 ppm. You need to use 37% formaldehyde gas in water/methanol. That is called "100% formalin". Do not use 10% NB formalin, that is a fixative.
To calculate the dose, you use the exact water volume of the tank (minus displacement for any rocks/gravel, but adding in the volume of any sump) and multiply that by 25 and then divide that by 264. So - if you had a tank that holds exactly 100 gallons, the math work out like this: 100 * 25 / 264 = 9.5 ml of formalin.
That said, I can almost ALWAYS find an alternative to using formalin.
Jay