I dose for the water volume, not the tank volume. No way to tell--it all depends on the quantity of other stuff. One of my 55's has about 49 gallons in it--very thin layer of sand, only a few rocks and wood, while the other one has about 42-43 gallons in it--thicker substrate, big piles of wood and rock. My 40 had a bit less than 30 gallons of water in it--very thick gravel layer, with rocks and wood all over the place.