Lookdowns are one of my favorite fish. I've been keeping them for more years than I care to admit. I've got three right now, each about 3 inches long. I caught them this past August, when they were very small, slightly bigger than a quarter. They can get big in the ocean, but 18 inches is pretty huge, even for a wild fish. I've maintained individuals that were tiny when I collected them for more than three years, at which point they were about 5 inches SL, about the size of a small dessert plate.
Lookdowns, Selene vomer, are members of the Jack family. They are extremely flat and thin, almost unbelievably so viewed head-on. They are a wonderful silver color, with a very steeply raked "forehead", thus "vomer". When very young, up to about 2 inches, they have long filamentous streamers from the dorsal and anal fins, much longer than the fish. When the fish gets bigger, the streamers are lost, but the silver acquires beautiful purple highlights. Sometimes Lookdowns are called Moonfish, and look a lot like African Pompano juveniles, to which they are related. Lookdows are fairly common in summer all the way up the east coast. They are very common in Florida.
Lookdowns are a little delicate at first when really small, but this is the best way to get them. They will adjust to captivity easily,and soon eat almost anything. Mine love Hikari "Marine A" pellets, but will eat chunks of frozen formula, worms, krill, etc., etc., and still more et ceteras. They are voracious, but have no bad habits, do not fight with other fish, and are spectacular in a large tank. They are a schooling fish, and there should be at least two of them. They are strong active swimmers, and a tank at least 4 feet long is necessary. Any fish that is much smaller is at risk, but I have kept big ones with basslets, chromis, etc., with no losses.
These are wonderful fish if you have the room, and can afford two or more. Around here, they sell for serious money, $75 to $100 each. I get mine for free, of course. If you have spotted any juveniles in north Jersey shops, there is a good chance I caught them.
I strongly recommend against getting a wild-caught adult. Juveniles are the only way to go.
