Risks with high alkalinity?

[aquarium]

I try to keep it at 7.2-7.6 to maximize the effectiveness of the chlorine. Which, BTW, I can keep at 1.5-2ppm (EDIT - 30ppm CYA) and stay clear blue. If you'll notice in the pic above, this pool is cut into a hill and I think the deep end is cooled by the ground, so it stays close to 84 degrees F, which might be helping.

[chem geek]

I try to keep it at 7.2-7.6 to maximize the effectiveness of the chlorine. Which, BTW, I can keep at 1.5-2ppm (EDIT - 30ppm CYA) and stay clear blue. If you'll notice in the pic above, this pool is cut into a hill and I think the deep end is cooled by the ground, so it stays close to 84 degrees F, which might be helping.

So here are the "calculated" numbers for your pool, just so we can see if theory matches reality or not.

At pH 7.2, the CO2 outgas rate is 45.7 which is very high; at 1.5 FC your ppm HOCl is 0.023; at 2.0 FC it's 0.032

At pH 7.6, the CO2 outgas rate is 17.2 which is possibly near "stable"; at 1.5 FC your ppm HOCl is 0.019; at 2.0 FC it's 0.027

So this implies you are having a strong tendency of rising pH. The theory would say that if you started at 7.6, then you would see a much smaller pH drift upwards and would use less acid. In fact, somewhere in the range of 7.6 to 7.8 could be a place where you would be about stable with little or no perceptible drift.

As you can see from the ppm HOCl numbers, they don't change that much with pH (due to CYA acting as a buffer for HOCl). You are at the "low" end of Ben's table range so proabably what he considers to normally prevent algae though he has given me worst-case examples with algae that were at about 0.05 ppm HOCl.

Even if you let yourself drift to a pH of 8.0, your FC of 1.5-2.0 would mean a ppm HOCl of 0.017 to 0.024 which is lower, but not by much. You could just increase your FC to be between 2.0 and 2.5 (or 3.0) and you'd be in pretty much the same shape.

I think the fact that CYA acts as a chlorine buffer to offset the effects of pH is a useful piece of information from the theory that would allow you to operate at higher pH with proper disinfection and algae prevention. If you are willing to take a small chance to try it out, I think it would be worth it given your high alkalinity and CH situation.

Now, as for the saturation index, yours is 1.0 and since you have no evidence of scaling nor cloudiness, that shows that the index can get quite high (we still don't know how high) before scaling is an issue. By the way, do you know your level of TDS? A higher TDS lowers the saturation index. Just based on your alkalinity and CH your TDS has to be at least 860.

Richard