Sad to say that is the one thing I just dont measure. I pour what I guess would be 100ml to my res. I heard one reason was to help limit alge and help oxygenate the water, but my air pump does a good job of that.. :D I dont know if I am getting a benefit becuase I have not ran without, and hell it is the second cheapest thing I add besides water... I got two bottles at Sam's club for less than $2. I dont see it hurting anything, and I dont want to see if it will.:thumbsup: