I wouldn't wrap fish to steam it. I'd only use foil if baking it.
I'll also mention, in passing, that firm-textured fish (such as fillets of salmon and whole trout) can be successfully cooked in a microwave. (Score the flesh of trout first. A couple of minutes, or so, on 'high' is usually long enough). Indeed, I'd suggest that microwaving is actually the best way to cook such fish. (It comes out beautifully moist).