but on another note i did find this stuff @ work i think may be suitable for waterproofing a tank. its some sort of fiberglass board, its thin, like 1/16 of an inch or so, its got some sort of stick backing to it to stick it to whatever, would just have to cut to size, stick it in place, and put a thick bead of silicone around all corners, and around the top. its used inside of industial coolers, so its FDA approved safe for the food industry. should be good for a tank also, just got to make sure to seal it super good. only thing is, the stuff is white, and i dunno if it comes in other colors. not a big fan of having a white tank inside. im going to call or go by lowes next day off and see if they carry the stuff and what colors it comes in. might try to get a sample from work to experiment with but there kinda strict about that stuff there.