That is the more extreme version of what my 40B was doing - a little salt creep along the bottom seam (only the top on mine). Kept thinking it was maybe from somewhere else, a little water during maintenance, etc., then wiping it off and it would reappear. I just replaced the whole tank and used to opportunity to upgrade a little. If it is the seam, you will need to replace or totally reseal the tank. It certainly looks like the seam, coming out above and below like that. I'd be concerned the whole thing will open up along that seam and work on a plan to empty the tank before it does so. Mine was leaking for several weeks and never got any worse until I replaced it, but then again it wasn't coming from the bottom at all.