Dose it right the first time — from your own readings

Enough to close the gap between your test reading and your target, scaled to the volume. Raising 10,000 gallons by 1 ppm takes about 10 fl oz of 12.5% liquid chlorine, or roughly 2 oz by weight of 65% cal-hypo — so one pound of cal-hypo moves 10,000 gallons about 7.8 ppm. Most pools are held between 1 and 3 ppm free chlorine.

Alkalinity, always. Total alkalinity is the buffer that keeps pH from bouncing around, so if you chase pH while alkalinity is out of range you will simply be doing it again next week. Get alkalinity into the 80 to 120 ppm band, let it settle, then adjust pH — it will often have moved into range on its own.

It depends far more on total alkalinity than on the pH reading itself, because alkalinity is what resists the change. That is why a pH figure alone cannot give you a dose. Add roughly half of any calculated amount, run the pump for at least an hour, then retest — acid is easy to overshoot and awkward to undo.

Usually stabilizer, sunlight or contamination. With no cyanuric acid in the water, direct sun can destroy a large share of free chlorine in a single day. Too much stabilizer causes the opposite problem — chlorine is present but sluggish. Somewhere around 30 to 50 ppm is the usual working range.

It adds calcium hardness with every dose, which accumulates and eventually scales heaters and surfaces in hard-water areas. Dichlor has the same issue with stabilizer, adding around 0.9 ppm of cyanuric acid for each 1 ppm of chlorine. Liquid chlorine adds neither, which is why it is the usual choice for routine dosing.

Not with a chemical. Cyanuric acid and calcium hardness both come down only by diluting — draining part of the pool and refilling with fresh water. That asymmetry is worth remembering when dosing: it is always easier to add a little more than to remove any.

No. Never combine pool chemicals, and keep concentrated products well apart in storage — cal-hypo mixed with trichlor, or any chlorine product with acid, can react violently and release chlorine gas. Add one product at a time, always into water rather than water into it, circulate, then retest before the next.