muraic acid does work. you need to throw something inwith it like nuts and bolts to scrub the inside.

BUT, 2 things. First, getting rid of the nasty stuff that comes out is going to be hard.
It will stain concrete. and I tried neutralizing it and was left with rusty sludge in a can. I threw it away after I was sure it woudn't react, but not sure what else is in it.

second, the tank will be raw metal inside again. so some sort of coating will be necessary to prevent rusting again.