You don't have to treat the stains before closing. Next year when you open the stains will be gone. When you get your pool going put in a metal sequestering agent, you will need more than a quart in a 25,000 gal. pool- follow the directions on the bottle. When you shock the water, you can add bleach on gallon at a time until you get to shock. If you see stain coming then add more sequestering agent. When you add the bleach, add it by the skimmer so the chorine goes into the filter before circulating. Keep the ph on the low side (around 7 - 7.2) while shocking the water. Get the pool water balanced. Don't add a lot of stabilizer so that the chlorine will be strong, and you won't need as much to clean up the pool also the low ph will help. You will learn to get a balance so that you can remain stain free without a lot of trouble.