There is nowhere near enough information in this post to make a proper diagnosis or treatment plan.
I am very sorry to hear that the fish has been lost but for future posters please know that we need as much information as possible about the fish and it's surroundings. Parameters (actual numbers) filtration, other inhabitants, feeding routine, anything that has changed lately no matter how small, and best of all, a clear pic or two.
I would not just assume it was Brook, though it may well have been.