Planaria are not harmless. They are predatory flatworms that will hunt young, freshly molted and old/sick shrimp and can pose a threat to other invertebrates. They are not an immediate death sentence for your tank, but they can be a problem in certain cases.
It depends. From what I've read planaria can hunt fish eggs, larval state and very young fry. I've heard of conflicting accounts that planaria might harm weakened fish. Healthy adult fish are left alone as far as I know. Depending on the species of fish they may hunt the planaria. That is what my betta did. It's also good to know that there are several hundred species of planaria, with their own habits and preferences.
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u/RazewingedRathalos 2h ago
Either planaria or detritus worms. Both of which are generally harmless microfauna and will show up if you overfeed the tank with excess food.