Are they real goonch catfish? Post pics for true confirmation in the catfish section from some of the more intelligent members on the species. If not, there are other species that look just like them that are usually sold as the true goonch catfish but rarely grow larger than 12". But if they are what you say they are then I would hope you know that a) they need a very large tank, and b) become VERY aggressive and have large teeth. The best thing that will incur growth is feedings of pellets high in protein, fish filets/whole fish that are species that do not contain thiaminase (bloodworms will not last you very long in terms of size) daily waterchanges, and slightly warmer tank temps. But goonch are usually considered a semi-tropical fish (like goldfish) meaning that they don't really live in really warm temps (75 is the highest I would go unless treating diseases.) I would also suggest adding a lot of oxygenation and water flow to the tank.
Quote from Wikipedia on the bagarius species:
"B. bagarius does not exceed much past 20 centimeters (7.9 in). B. rutilus grows to about 100.0 cm (39.4 in) SL. B. suchus grows to about 70.0 cm (27.6 in) SL. B. yarelli grows very large, reaching about 200 cm (78.7 in) SL."
