What is the proper way to brine fish?
It depends on why you're brining. I don't smoke fish so longer brining may be warranted for that use. I brine to keep fish juicy coming off the grill. For this I agree with Cook's illustrated:

We found that, for up to six 1-inch-thick steaks or fillets, the optimum concentration was a 6 percent brine (5 tablespoons of salt dissolved in 2 quarts of water) and the ideal time was 15 minutes. It worked no matter the species, improving the texture of the fish without over seasoning....
And brining works a lot faster on fish because the structure of muscle in fish is different than that in meat: Instead of long, thin fibers (as long as 10 centimeters in meat), fish is constructed of very short (up to 10 times shorter) bundles of fibers.