Ez way http://www.aqua-calc.com/calc_vap.php
US gallon149.61039 So 150 is correct.
And although Useless for our size tanks this is always nice.
Fresh Water Sea Water
Pressure at the bottom of the tank, psi 0.86 0.89
Total (uniformly spread side to side) force along the bottom strip of the tank pushing outwards against your joints, lbs 62.21 64.08
Total force on the long side, lbs 622.08 640.80