With a muriatic acid solution you'll need to fill the tank completely to the lip for it to work properly and let it sit for at least a few full days , flush and repeat with a slightly stronger solution. Just sloshing it around will cause rust to acclerate where it is not completely immersed as it will be exposed to air. Remember it's FeO2 , rust needs oxygen. immersing it allows the acid to reverse the loose bond the rust has with the steel. Always wear a charcoal filtered mask around muriatic acid and have a hose ready to rinse for safety.