Given you already operate a restaurant, you have a daily head count of meals served. Will the new operation increase business? Start by estimating how many customers would order catfish. As Tom pointed out, a fryer would also provide the means of adding French Fries, and perhaps onion rings, fried okra (
arefrom Louisianna
)and whatever else you might deem sales worthy. The end game is to come up with a general idea of how much revenue the investment of a hood system will generate. In addition to the cost of the fryer(s) and hood (which may require a "make-up" air system, depending on local/state code) you'll need a fire suppression system.
Then there is the cost of fryolater oil. I pay $34 for a 35 lb container of trans fat free Canola oil...which will fill one 35 lb fryer. Depending in volume and use, you'll need to strain the oil once, if not twice a day...and replace the oil every 3-5 days, Then there's the cost of having the used oil removed...and the space needed to store a grease barrel. Some outfits were paying a few bucks for used oil back when gas was $4 @ gal.
Adding catfish to your menu sounds like a good if not great idea...broadens your appeal to a wider customer base. Let me know if I can be of further assistance.