all i know is i've got a 29 gallon, 30lb of rock and about 2 inches of sand. took it down to 20 gallons and 80 drops of the stuff put me exactly at 2ppmPosted a picture in another response. It was Dr. Tim's Ammonia. I only used 2/3 the recommended dose. I doubt the sand and rocks displaced >33% or 25 gallons.