Another reason why this does not work to ask this question is because of the variables of rock and sand displacement In the actual tank. So one persons 100 gallons to 20 lbs of salt may be another persons 92 gallons and 18lbs of salt due to having more rock. Anyways hope this answers your questionSetting up a 120g saltwater tank and what's the ratio of salt to water to add? If that's clear enough lol