Fish in U.S. zoos

Fish appear in most zoos as supporting cast — a stream tank in a rainforest building, a touch pool, the underwater half of an otter or penguin exhibit — rather than as collections in their own right. That changes the moment you cross into aquariums, which are a different institution type with different standards. Where a zoo does invest in fish, the tell is water clarity and the health of the planting: a well-run freshwater system is visibly different from a neglected one, and it's one of the easier things for a visitor to judge.