What are you trying to measure? The volume per vertical inch? Easiest method is going to be to remove a known volume (ie, 5 gallons) and then measure. Bow tanks are a bit trickier to calculate, but you can use the basic formula for a rectangle and then divide in half (ie, .5(w X l X h) to get a ballpark figure. But measuring it as you remove is going to be the best way, since you don't have to account for displacement of water by objects (filter, rock, wood, plants, etc.
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~Sheila
tempus edax rerum
Ideas do not have to be correct in order to be good; its only necessary that, if they do fail, they do so in an interesting way. ~Robert Rosen
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