FORUM RULES UPDATED - The forum rules have been updated to continue to provide a friendly and safe environment for our member community. Please take a moment to review the updated rules and our terms of use and privacy policy. Click here for Forum Rules which has links to the 'terms of use' and 'privacy policy'.
If the tank is square/rectangle you can measure it and multiply L xW xH (in inches) and divide that number by 231. That should get you pretty close to how many gallons the tank is.