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My brother seems to think that coins would be a great substrate, now this definetly wouldn't work for a turtle, but he does have a 46 gallon bowfront with fish. Any chance just boiling them would work or would a chemical reaction be too bad for the water quality? He only has three fish so the cleaning wouldn't be a major problem.

Logically that seems like a poor idea to me. I'm not sure what exactly would or could happen chemically, but it seems like a risky idea to me.

Yeah, boiling would definitely get rid of the germs and all, but I'm unsure of the metal. I think it would look good but I can't find anyone who has ever tried it. He got bored of the gravel and doesn't want marbles and doesn't know how to get sand down properly.

I would vote against coins underwater, as they will start to give off copper/iron/silver/whatever into the water which is bad for the fish and bad for the coins. If you want to go for something weird and different, maybe find something like galvanized steel nuts and bolts for a nice industrial setting, or go through various craft sections, like some super Walmarts have nice sections with a lot of rocks or glass drops that can be used in different combinations to create a nice look.