I want to build a plywood fish tank 16' long x 4' wide x 3.5' high. I need to know how to calculate the lateral force at any point on the walls in order to build with strong enough lumber to support the lateral pressure and also the weight pushing down. I know that one gallon weighs 8.345 lbs. and one cubic foot has 7.48 gallons, so it's not too difficult to calculate the weight pushing down. The lateral force is where comes the problem.