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I think I read somewhere that keeping fish in the tank can cause parasites or other bacteria. Is this true? I recently got some rosy red minnows.

Im sure they can if the fish have parasites that will grow in a turtle. Like some sort of worm or something. I think that many people have fish in their turtle tanks. They can help clean up the mess that the turtles make.

it can happend which is why it is good to quarantine fish when you get them. alot of times you can tell just by looking if they have a parasite or a growth. those are the ones you toss out

Yes they can. The common practice is to quarantine new animals for at least 60 days. This is done to check the new addition for any illnesses and/or parasites and to prevent their transfer to the established tank.

If the fish have parasites and are in the same tank with turtles, the turtles can eventually get them as well.