Baking soda is a temperary solution, at best. Adding limestone rocks to
your water flow is the best solution, IMHO. By the way, alkalinity has
little affect on string algae, since it is present as a result of high
nutrient load in the water (aka, high nitrate concentrations).

Alkalinity does have an affect if the pH is too high or too low, most water
plants that we use take up nutrients best when pH is 7.2 to 8.2, even the
bacteria in our filters and our ponds work best between those numbers.
Thus, if your pond is 8.5 and the desirable plants are not absorbing the
majority of nutrients, one is going to have more problems with algae.
There is always some type of algae that can live in whatever extreme our
ponds are in, but they can't survive (as well) without a food supply. ~ jan

~Power to the Porg, Flow On!~