Raphael. they are not really a newbe fish. They should be fed live black worms, small pieces of fresh clam, mysis, plankton etc. They get about 5" long but they stay stunted in a tank and may grow to 4" depending on how large the tank is. You can keep them in a 50 gallon tank but of course bigger is better. If the tank was just set up I would wait a year or so.
They were always my favorite fish and I have been keeping them for over 30 years. Make sure it is eating in the store, a lot of them are caught with cyanide and do not survive long.
Paul
Here's mine
