: What is this parasite? I found this parasite in my tank this week. It was the second time I saw it but the first time I lost him before I could catch it. He wasn't on any fish or plant, he was on a wood piece the first time

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John's answer is spot on. I'm pretty sure it was effectively a damselfly larvae.

I want to add about how it possibly got in the tank and how I watched to tank the following weeks to ensure I catch them all.

First, shortly before I saw this little guy for the first time I had purchased 2 new plants. As John pointed in his answer the larvae probably came from a plant. In my case it was a Java fern and some Whorled Pennywort - Hydrocotyle verticillata. From the plant formation it would have been pretty hard to spot them in store but once in my tank they didn't stayed on the plants. I saw them on wood, sponge filter, rocks, etc.

For the quantity, I only caught two. One when I posted this question and another one like 2 week after the posting of the question. So my advise here is if you found one be sure to keep watching for some time after because there are good chance it wasn't alone.

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Looks like a damselfly larvae to me, which isn't actually a parasite. It is, however, carnivorous...

If your fish were larger, this larvae could have turned into food for them, but not in this case as you have small fish. So, as it is they can be dangerous to your fish and shrimp, so best to make sure you get them out of the tanks as soon you find them.

Not sure how it got to your tank, but best to be careful and quarantine new plants for your tank for some time to ensure that they are free of unwanted guests.