This seems like a good idea to raise the borate level to the 50 ppm level and creating an algistatic state. I found a patent from 1986 claiming borates are also algicidal: http://www.freepatentsonline.com/4594091.pdf
(does such reading belong in "The China Shop"?)

One last thing 20 mule team borax comes in 76 oz. boxes, coming to 4.75 lbs., so if you overshoot the dose is there any harm in putting in too many borates?