Epoxy putty and backgrounds

Hi There i am one of those ppl That makes customised backgrounds for aquariums. Now i want to experiment with epoxy putty to sculpt trees. I know there is special aquarium putty on the market but it is very expensive. I saw some epoxy putty that is used for repairing pools. This comes in larger quantities and is less expensive. I'm very much limited where to buy putty cos I live in the Netherlands. I have informed about the product and they said once cured it is 100% save for humans but they didn't know whetet it would be save for fish. Do u guys have any ideas about this? Thnx