What chemicals do I actually need to open a pool?

You only need three: shock (non-chlorine or chlorine), algaecide, and stabilizer (cyanuric acid) if your levels are low. Shock kills winter growth, algaecide prevents new algae, and stabilizer protects chlorine from the sun. Test and adjust pH and alkalinity first, but those are water balance, not opening chemicals. Don’t buy overpriced opening kits with extra bottles.