Hi and welcome! Stainless steel is very inert so I would think it's pretty harmless, however I don't know how it would work in a pond that is outside. A few issues are algae may not be able to coat the inside properly like you would have on an epdm liner so the nitrogen cycle may not function the way you would normally want it to work. Maybe the biggest problem is the liner may get too hot in direct sunlight and raise the pond water higher than is safe for your fish or plants for that matter too. Another problem I see is it would reflect sunlight too much and that could be a problem. Maybe you would have to buy your fish sunglasses . I'm guessing they probably use steel for indoor use for aquaponics or hydroponics, but it doesn't seem to make sense for an outdoor pond.