[SantaMonica]

Without dosing alk, the slow process of CO2 dissolution into the water has stabilized with the algal uptake, in your case, at 125 ppm. CO2 is going into the water (from fish too), and then to the algae. What you don't see is the rate of transfer; it is very high, like a flowing river. But the river is shallow.

Algae would grow faster if more alk were available; that it why when you dose, it is used up right away. Extra rates of growth may be noticed or not; it may take two days to double in size instead of one day, and you still only notice the final size, but the increase in rate would be a 100% faster rate of uptake.