We have a 25,000 gal Concrete pool in NC and the water is so dark green, it's borderline black!
Ordinarily we'd drain it, however the water table is within 1' of the surface, so that's not an option.
Here is the chemistry before we started last weekend.
Hardness 200-250, PH 8.4 !, Alk 120, CYA 120
Thus far 42 pounds of cal hypo has had little effect and we still can't see the bottom.
We'll be flocking today.
The plan is to vacuum to waste using the skimmer connection to clean the algae from the bottom, as there is no main drain on this pool.
Once we've vacuumed the bottom, we'll plan to drain down about 18" and refill to lower the cya.
We understand that the chlorine added has limited effect due to the cya. we have seen that chart.
PS. We read somewhere that cya may typically reside on the top water surface. Anyone tested this idea to see if it is true? If so, possibly we should not pump from the bottom of the pool. It seems more likely that cya would be intermixed throughout the entire pool!