little shrimp? ish looking critter

when i turned on my tank light on at night after being off for awhile, i saw little shrimp ish looking critter hanging around at sand bed. It was maybe like 1-2 millimeter big, size of small sand pebble I guess. There were like 5-10 of them just at the front of the tank, so probably more through out the tank. At the back of the shrimp, it had black and white ish stripe, and had a half circular body of 180 degree, basically a circle cut in half shape like this (

Sorry i suddenly forgot the word for it.

And it had many legs, they didnt dissapear suddenly, but they were acting and moving fast, but after 5 minute ish later, they eventually dissapeared to somewhere while I went to find my camera.

Anybody know what they are?

 

Amphipod

 

Amphipods Mkizla

Steve

 

are they okay or bad?

 

great for the most part, there are some that are in the ocean that wil eat fish and are pretty wicked but getting them in a tank is pretty rare.