Oh... well i know that there is a product out there to line concrete ponds (as in fish ponds or water gardens). I think its "painted" on... maybe sprayed on? I do not know the name of it though. I would think if its fish safe then it would be ok but that is just my opinion. I would search through google or whatever search engine you like to use and see if you can find the product then contact the manufacturer about how you want to use it. Maybe they can help?