You add water conditioner any time new water is added to the tank. There are products such as Stress Coat which is basically a water conditioner but claims to also help when new fish are added by protecting their slime coat. I've never personally used it and feel it's unnecessary.
When adding new fish it helps to keep the lights off for a few hours after they are added. Be sure to acclimate them first bu floating the bag in the tank and adding some tank water to the bag every 5-10 minutes or so. Most importantly is to first quarantine any new fish in a separate tank for at least 2-3 weeks to be sure no illnesses are passed on to the fish in your main tank.
