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Somebody told me they can actually keep the tank clean by eating stuff in the gravel.

That would make sense. As long as it wasn't the dismembered parts of the other tank dwellers. My friend bought a blue lobster once, no research or anything. Popped out in his tropical fish tank. Two days later the murderous shrimp they it actually was destroyed everything else that lived in his tank. He was a bit taken aback!

I have heard about these lobsters, apparently they eat anything they can catch. (They are actually illegal in South Africa, but as all things illegal, one can still get them, they are apparently invasive)
These little shrimps on the other hand will be eaten by the fish, that is why I keep them separate from my fish.

Yeah they are bd, its the same here, people will get them then release them. Yeek.

They are cute little shrimps!