I used in mine muriatic acid! Fill the tank and keep it there for 15 minutes, then drain(before start remove petcock first and use an plastic stopper at hole, haves to be very well sealed). After drain all liquid If needed repeat the process many times until all rust have been removed. Be very carefull working with this acid, use long rubber gloves, mask and security glasses for your protection. This process will create an nocive gas, just can be made in an very ventillated place or outdoor only. Attention to paint and floor, this acid is very corrosive, will damage all kind of surfaces.