What flea thingy.....

Pod. Free fish food and a good thing.

 

I didn't realize they would get that big in captivity.

 

i see some in my tank from time to time that are at LEAST that big... but they get eaten pretty fast

 

Well that's probably why I've never seen one that big...they get eaten so fast.

 

We've got some quite large ones in our refugium.

 

+1 on amphipod, but I'm sure you did not need confirmation at this point.
They're very good small crustaceans that will eat detritus and will also serve as fish food as everyone mentioned above.

I have found some in my tank that are at least half an inch. They can grow too big for fish food, depending on the type of fish you have, but they will always make babies and it's a good thing. Great biological clean up crew.

 

I need to get somethign to eat mine up they are crawing over everythign in my tank.