Is this a 'nasty' in my tank?

I was just cleaning out my sock style prefilter in my sump. I had a bunch of cherry shrimp in there so I dumped it into a small container and then picked out the shrimp. But I noticed something else in there much smaller than the cherries, and different shape. At first I thought it was a baby cherry but it just didnt look like the right shape. I thought of the macro lens I just received the day before so I got it out. So technically this is the first macro shoot I've ever done and I know I have a long ways to go to get decent pics. But hopefully its clear enough for ID. I wasnt a good setup, I didnt have a tripod so it was hand held, and the ambient room lighting was dim so all the light is from the pop up flash on the camera. Approximate size was 1/4" at the most. Of the group of about two dozen cherries I got out of the prefilter there were at least a few of these.

I'd like to know what this thing is and if it is anything to worry about. Thanks for any help.

Yeah after googling I do believe they are amphipods of some sort. But most of the results werent aquarium related so I am unsure if these are bad in the aquarium? Are they anything to be concerned about? I havent actually seen any in the tank, just the prefilter in the sump when I cleaned it out, so its not like I have an infestation. However in my 240g tank... that would be a lot if an infestation is just around the corner...