Don't mix chemicals — add them one at a time, in this order.

Alkalinity first, then pH, then chlorine, then shock, then algaecide. Wait at least 30 minutes between each. Never add two chemicals at the same time, and always add chemicals to water, not water to chemicals.

The main danger is mixing chlorine and acid — that creates toxic chlorine gas. Even if you’re careful, if you dump shock into a bucket that had acid in it, you’re in trouble. That’s why you do the balancing chemicals (alkalinity, pH) before the sanitizers and oxidizers. Alkalinity and pH work together anyway, so adjust them first.

Also, don’t add algaecide right after shock — the chlorine can break it down. Wait until chlorine levels drop below 5 ppm. And never, ever add dry chemicals directly to the skimmer. Pre-dissolve in a bucket of pool water, then pour around the perimeter.

The order is simple rule: Balancers → Sanitizers → Oxidizers → Algaecides. Stick to it and you won’t gas yourself.