“Junk” fossils that you don’t mind if they get algeaed up, etc. are great displayed in an aquarium. In fact, it seems to be their natural home as all the fossils from the Ordovician are of sea creatures.
If you are concerned about the health of your fish by adding limestone fossils to the aquarium, just do an ocassional pH test and adjust if needed. For goldfish, koi, (basically carp) and bettas I have never had a problem.
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