What dilution fixes — and what it doesn't
Three things in pool water have no chemical exit: cyanuric acid, calcium, and salt. Once they're high, every bottle promising otherwise is either acid in disguise or a sequestrant that hides the problem on a subscription. Water replacement is the fix, and because one drain serves all three, it's worth planning them together — this calculator takes whichever readings are high and finds the single percentage that lands everything, driven by whichever parameter needs the most.
What dilution is not for: pH, chlorine and alkalinity all have cheap chemical adjustments — draining water to chase them is paying for water you didn't need to move. And if only one reading is high, check its own calculator first (CYA, CH, salt) — sometimes the honest answer is that the number isn't worth a drain yet.
An empty pool is a fragile pool. Vinyl liners relax and float the moment water stops holding them against the walls; fiberglass shells and even concrete pools can lift out of the ground when groundwater pushes on an empty basin — "popping" a pool is a five-figure mistake. Keep at least a third of the water at all times: a 60% change done as two ~37% drains achieves the same dilution as one 60% drain, because each stage works on already-diluted water. The calculator shows the staged plan automatically for anything over 40%.
The no-drain option: fill and drain at the same time
If a liner float or a high water table makes any real drain too risky, run the hose in while a pump takes water out at the same rate — the level never drops. The trade-off is efficiency: with continuous mixing, new water dilutes old water that's already partly new. Cutting a parameter by 60% takes about 0.9 pool volumes of water this way, versus 0.6 volumes as a batch drain — roughly 50% more water for zero structural risk. On a 15,000-gallon pool, that's the difference between 9,000 and 13,700 gallons; whether that premium is worth it depends on your water rates and your nerves.
Drain when the refill is cheapest and safest: cooler weather (less evaporation), after checking your utility's rates (some bill sewer on water metered in — a pool-fill credit request can halve the bill; the cost to fill calculator prices the drain with and without it), and never right after heavy rain if you're on a high water table. If you're paying for hauled water or on a well, the staged approach also spreads the load across days.
After the refill
Fresh fill water brings its own chemistry: typically some calcium and alkalinity, zero CYA, zero chlorine. Retest everything a few hours after topping off, then rebuild in order — chlorine first (the refilled pool is unprotected), then pH and TA, then re-stabilize. The dashboard handles the whole restart from one set of readings. Expect to re-dose salt if you drained an SWG pool; that's not a failure, it's the plan working.