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Measure the length, width, and heigth of your tank. Multiply those 3 numbers together to get the volume (if you measure in centimeters, you get a measure in cubic centimeters or "cc". 1 cc =1 milliliter, or 1000 cc = 1 liter). Once you have a volume in liters, you can convert it to gallons. I'm not sure, but I think 10 liters is about 2 and a half gallons....probably your best bet is to find the exact conversion online!