Mr400 had a concrete tank and he used a pool sealer. Check out his site for more info mr4000.com
Here's a line from the Q&A section of his site
Q. What did you seal the tank with?
A. I found that you could use pool paint so i got 12 year epoxy paint.(they said it should last longer than 12 years since it's not exposed to the outdoor elements) i hope there right!