Help with identifying this from my filter sock

[Idoc]

I found this in my filter sock tonight. Its about 3/8-1/2" in length and very active. It curls its back tail up under itself and seems to have front arms/legs held tightly to its body like a pistol shrimp or mantis shrimp. Kind of swims on its side... or maybe that's because I don't have much water in the cup!

And then the next question...keep it?

Does anyone have some good identication site links to look things like this up?

Same critter, just in a different colored container:

 

[Idoc]

Here is another pic through a +30D condensing lens.

 

[cmcoker]

Amphipod, they are fine.
https://www.lionfishlair.com/hitchhikers-guide/amphipod-gammarid/

 

[Idoc]

Amphipod, they are fine.
https://www.lionfishlair.com/hitchhikers-guide/amphipod-gammarid/

Thanks! I just put him back in the tank!

 

[Salty1962]

Good pat of CUC and food source for livestock.