We buy our tanks. Let's say a 29G tank from the store for example. You do your measurements and ya, it will hold 29G of water. Great. Then we start thinking about adding plants, decoration, filters, pumps, fish etc, etc. Is the tank still 29G? When you consider displacement with all these added components, we no longer have that 29G empty tank that we first purchased. So when you think about stocking, medicating, and all that good stuff. I think it's important to keep in mind that we are actually dealing with a smaller tank then we think we have.