dude, dont use hardi board. just lay fiberglass and lots of resin then sand the resin and epoxy the thing. you are gonna add so much weight to the tank and the leaks if any will spring up from the joints which the hardi board wont do anything for. if you already started then good luck. i am a contractor and work with this stuff all the time. hardie cement board seems good to me to but id be worried about long term water seepage. water is one of the most distructive substances to deal with. and it eventualy degrades everything it comes in ccontact with . pics please!!!!