I believe nolacola is talking about the Violet Goby, otherwise sold under the name "South American Dragonfish"
They reach a size of about 2 feet in length, so would need a large tank...I would say 75 gallon absolute minimum.
They are not aggressive unless the bottom of the tank is overly crowded with too many bottom feeders...then they can infllct some nasty wounds on other fish as they have tiny sharp teeth.
Their eyesight is very poor, almost blind. You may have to target feed them with a turkey baster or something similar to get the food close to him so no other fish get to it first.
Violet Gobies are brackish water, meaning they need a higher salinity. You will have to mix marine salt in the water to get brackish conditions. I'm not sure what the salt to gallons ratio is though...varies depending on the tank size.
These guys eat live and frozen foods like, bloodworms, brineshrimp, beefheart etc. They are filter feeders, so they need a sandy substrate or a very fine gravel bottom, as they can choke on the gravel if it's too big to pass through their gills. Usually when they're good size, you shouldn't have to worry too much about that. They may eat smaller fish, but only if the smaller fish is dead or dying...their eyesight is far too poor for them to be able to hunt/chase anything down.
Don't keep these guys with aggressive fish as they tend to get picked on and harrassed since they just lay around for most of the day. Dragonfish are nocturnal, so most activity will be at night after lights out.
Try google for more info and look under the name Violet Goby.
Good luck and let us know if u have any other questions! :thm: