12 oz. of borax raises 1000 gallons 10 ppm.

To reach 50ppm required 60oz of borax per thousand gallons.

Then 21K gallons = 1260 oz of borax. Each box (in my area) has 76 oz, which is roughly 16 and a half boxes.

You also have to add 6 oz of 31% Muratic acid for each 12 oz of borax. That means a bit less than 5 gallons of acid.