that would be getting RID of suspected soap residue...sorry. anyway! I just bought a 20 gallon off a guy from craigslist. tank looks in good shape but I think he cleaned it with the soapy mixture I saw him cleaning his truck with when I pulled up. I didn't say anything but was kind of disapointed. is there a good way to ensure all that residue is gone before I try to set this thing up. i'm gonna test it for leaks tonight...