How much chlorine do I add to a 20,000 gallon poolWelcome to TFP! Switching to chlorine is easy - just add it. But the first thing to consider is how much residual product from the PS system was added to your water (i.e. copper)? I suspect the copper level is high if you are seeing (copper) staining. If so, you should consider exchanging some water first. If you do, remember to not drain the entire pool. Always retain at least 1 foot or two to keep the liner & shell stable.
Next question - how are you testing? You want to have a TF-100 (or Taylor K-2006C) test kit on-hand. That's a must. See Test Kits Compared. Avoid worthless test strips and unreliable free testing at all the local places. Then run your own full set of test numbers. Compare to the TFP Recommended Levels - Trouble Free Pool page for best success.
You can get liquid chlorine (aka Regular Bleach) from just about anywhere in our area. I used to get HEB's Bravo regular until they lowered the strength (at the same price), so now I grab chlorine from the local hardware store. But shop around in your neighborhood for best deals.
Make sure to review our ABCs of Pool Water Chemistry and let us know if you have more questions.