121oz. of 8.25% bleach should add about 5ppm FC to your 15K pool.

How much to dose depends on the measured FC level and how much you predict the pool will lose before the next FC measurement / dose.

Let's assume a 3ppm FC loss in a day and the last measurement is 3ppm FC. Three 121oz. jugs of 8.25% will add about 15ppm FC raising the FC to 18ppm. if the pool loses 3 ppm tomorrow, the remaining FC should be 15ppm - the minimum shock level. Then add a bit more than a half a jug to get back to 18ppm so the FC will be at least 15ppm the next time. NOTE - this is only an example - you'll have to put in your pool's loss rate

It's important to remember to measure - dose - measure and adjust as necessary. The more often you do this during the shock process, the better it will go.

As for the wire brush - you ultimately have to make that call. perhaps someone here will recognize your pool's surface and chime in but I have a vinyl and wouldn't go near it with a wire brush.