What should I do after adding a lot of fresh water to my pool?

Treat it like starting over — test everything from scratch because fresh water diluted all your chemical levels. Check pH, alkalinity, chlorine (free and total), stabilizer (CYA), and calcium hardness; each likely dropped by the same percentage as the water you added. Add roughly 30% of what you normally use if you replaced 30% of the water, and don’t forget stabilizer if you use chlorine tablets. Shock the pool if you added a lot at once, and adjust in order: alkalinity first, then pH, then chlorine/shock, then anything else — wait a few hours between adjustments.