Doh! I forgot to carry the 2. Anyway, if you follow the advice above and measure the water level drop after removing all of the items from the tank, you would have just calculated the volume of all of those objects. Not very helpful for fert dosing, I know. You would still want to start with the water at max level. Then when everything is out, you would use a clean measuring tape to measure the water column height from the bottom of the tank. Using this height in the above calculation, with the appropriate conversion, would give you the volume of water in your fully decorated/planted aquarium.
If you don't want to worry about the filter volume, just exclude that step. Also, this method can be used to determine the volume of water one inch of water column height equals in irregular shaped tanks (bow, hex, corner, etc.). All you need to do is put a mark at the current water level (are you measuring from the meniscus . . .) Then you would measure down one inch and put another mark. Now drain that volume of water out and measure it with some precise measuring device. Five-gallon buckets work nice for this if you can get the whole inch into it without overfilling it. Then just use the simple formula for the volume of a cylinder: pie are round, not squared . . . times the height of the water in the bucket and you have that 1-inch volume for any complex shaped tank. If 1-inch is going to be more than 5 gallons for your tank, you can just measure the gallons required to raise the water back that 1-inch using some trusted 1-gallon container. Now think about it, umkay.
