To find the NET Gallons use inside dimensions and measure the height to the top of the water line only (to the bottom side of the lip found on most aquarium frames). The formula is L x W x H x 0.0043 for rectangular aquariums.
Of course this will be less after addding gravel, decor and plants. So, I always add my water to my tanks by a set quantity of gallons and keep track, then write it down for future reference when adding any chemicals, fertilizers or medications, etc.
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