They sound like encysted metacercaria of a digenetic trematode. They usually
only cause minor irritation if any, and fish can live and breed quite
successfully while infected. There is little chance of transmission to other
fish, as what you are seeing is just an intermediate stage on the way to the
adult fluke ending up in it's final host, usually a piscivorous bird. Just
keep your pet Heron away from the tank and you should be OK.
They are found pretty well all over the world, I've seen them on fish from
here in the UK, South America, Asia and West Africa.
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