Holy cow.... I think you just helped solve the algae mystery that’s plagued this pool for years and years. Now that I think about it, you’re absolutely correct. Those basins fill up with so much water and it just sits there, even when the waterfall is on! They builder should have put return jets at the bottom of each of these basins to inject fresh return water and to push it up so it will overflow into the next basin. That HAS to be the reason why this pool would always get algae. Even with an appropriate amount of chlorine level, as soon as the waterfall is off and it sits, that chlorine wears off in the water in the basins and when it gets turned back on and if/when that water eventually makes it into the pool, it has the algae.
Now I’m just going to have to play with it to make sure I can still fill up the basins with concrete but still create a good waterfall flow effect. One of the top basins is very long and wide because it has water coming from 2 sides. I wish they made some sort of temporary waterproof clay that I could plug this thing up and to see how the water will flow with smaller basins.
