So I bought a used 20 gallon long today. I have a suspicion that they used soap (or something like it) to clean it. The tank has a faint smell that I didn't notice when I picked it up. So is there any way to make the tank safe for use?
I would recommend using swimming pool chlorine. A lot of the bleach in the normal retailers contain surfactants. If you have crud to scrub off the walls, use uniodized salt. After a thorough rinse, put it in a sunny place for a few days to expel the chlorine. When you begin to fill, do so slowly to check for any leaks.
The tank is actually very clean (one of the reasons why I figured there was soap in it). I called the person I bought it from and it did have soap in it and I'll be using bleach to get it all safe tomorrow. Thanks for all the help.