Different brands of salt have different procedures. Some have more trace elements, and need to be used between 20 minutes - 4 hours of mixing to keep everything dissolved in the water, while others can sit around. Iirc IO is ok to store if you want to.
Always add the salt to the water, never water to salt.
Always replace evaporated tank water with unsalted water.
Depending on the size of your tank it can be muuuch cheaper to buy a rodi filter for your water. Tap water can have chlorine and metals that water conditioners handle, and also nutrients like phosphate or elements like alkalinity or calcium that will throw off the balance of your salt mix. Water conditioners don't touch those. The salt is like evaporated ocean water, so it is developed to have pure h2o added back to it and arrive at the proper balance. If your tap water has alk and Ca to begin with, and your salt adds it too, you can wind up with too much.
If you can't swing a filter just yet, "distilled" water is a pure substitute, not "drinking" or "spring" bottled water. You can get distilled from Walmart for like $1 a gallon. I wouldn't buy water from a fish store. There's a lot of horror stories about them being lazy changing filters and stuff, but it's impossible to test the water for purity once the salt is added, so you just have to trust them.
Welcome and good luck!
