Is it normal for my tank to bulge on the bottom sidewall when I fill it up?
Yes. Polyethylene, by its very nature, is a flexible material. Flexibility is an inherent strength and helps compensate for protection from impact, deflection, water hammer or surge, temperature variations (expansion coefficient), etc. This is another reason that it’s important to use expansion joints or other flexible connections when designing your piping system with your tank.
Source: FAQs - Peabody