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Goldfish are a favorite ornamental fish and can live for over 40 years. But telling how old is difficult to do except with a microscope. The technique involves examining and counting the closely spaced rings of the scales, similar to how tree rings can be counted to tell the age of a tree.
Microscopic observations of fish scales are also used for species differentiation, and some scientists study fish scales as a monitor for the environment.
See all Microscopes from Explore Scientific