Definitely a silver scat. I had a few of them before they outgrew my tanks and I traded them back to the LFS. They are schooling fish and should be kept in groups of six or more. They are often sold at the size of a quarter but will grow extremely fast. They get to be about 12" in length and are very active. Often, they are kept in and sold as freshwater, but they should be kept in increasingly higher brackish water as they age, ending with full marine conditions when they are adults. They are voracious eaters and will hog food from anything else in the tank.
I would really like to have a school of them at some point in the future but I wouldn't recommend getting a group unless you have at least a 10' by 3' tank. They need a lot of swimming space.