Treat it like starting over.

Test everything again and adjust from scratch. The fresh water diluted all your chemical levels, so don’t assume anything is still balanced.

The big ones to check: pH, alkalinity, chlorine (free and total), stabilizer (CYA if you use it), and calcium hardness. Each one likely dropped by the same percentage as the water you added. For example, if you replaced 30% of the pool water, all levels are roughly 70% of what they were. That means you’ll need to add roughly 30% of what you normally use to get back to target ranges.

Don’t forget stabilizer (cyanuric acid) if you use chlorine tablets — it’s easily overlooked and hard to fix later. Calcium hardness matters more for plaster pools than vinyl or fiberglass, but still worth a quick test. And if you have a salt system, top up the salt too.

Shock the pool if you added a lot of water at once — it’ll kill anything that might have been stirred up from sediment or old pipes.

Do it in order: alkalinity first, then pH, then chlorine / shock, then anything else. Wait a few hours between adjustments.

This is not a time to wing it. Test strips or a drop kit are your friends here.