Coin Decoration

[Wolf13542]

I am making a giant coin decoration for my fish tank. Like making a huge nickel or something to put in my tank. I looked up tinfoil and coffee can bottoms but those are toxic. The theme is the botttomo of a pond and I have large rocks and gravel, a ceramic shell and snail and plastic plants. The tank I have is a 10 gallon freshwater tank with a 5 inch featherfin synodontis. What won't corrode or poisin my fish?