Will my rectangular tank bulge when I fill it with liquid?
Rectangular plastic tanks do not have the same physical properties as fiberglass or steel and they are not designed to hold heavy loads on their sidewalls without support. Properly installed rectangular plastic tanks will give you years of trouble-free service, but you must support the tank with an external steel cage or a fiberglass overwrap. We can provide either option for you if you like. Most of our rectangular tanks have the standard fiberglass overwrap option listed on our website. Generally, tanks shorter than 18″ tall do not require sidewall support, but if you’re not sure, check with us before ordering.
Source: FAQs - Peabody