You got a picture? Ageneiosus Brevifilis is a synonym for Ageneiosus inermis, which gets to about 21". I'd expect it to be OK for a week or maybe two, depending on it's condition when you bought it. If it's wild caught it won't be used to prepared food. What are you feeding it? I'd suggest something like live river shrimp, but if you'd rather avoid live food, loosely tie some cotton to a fresh/defrosted prawn/shrimp and try teasing the fish with it, with the lights out and see if it will take it. If you intend to not feed it live, then you need to start with this approach straight away - they will usually feed once they are hungry enough. Keep the lighting subdued, and the water clean, give it lots of places to hide, and it should be feeding soon.
Good luck and keep us informed.
Martin
thank you martin! i saw him finally eat i fed him feeders i seen him eat one amd he got some gravel in his mouth and when i checked on him later on lastnight his belly was full and this morning his belly was back to normal. will he be ok eating a little gravel will pass through?