Without a decent test kit it's very hard to maintain your pool. We suggest the K-2006 from Taylor. There's a link on Ben's and Watermom's signature to Amazon that helps support our site...and the price is competitive.

Generally, for a 20,000 gallon pool (close enough), 1 gallon of 6% adds 3ppm of free chlorine. 5.25% adds 2 5/8 ppm. 12.5% adds 6.25ppm.

The object of the exercise is to not have to add chlorine more than once every couple of days, but to do that, you need to have Cyanuric Acid in your pool, called CYA, Stabilizer, or Conditioner. This helps keep the chlorine from breaking down but slows down its processing as well. But the more stabilizer you have, the more chlorine you need in you pool as a base. We have something called the "Best Guess" table. Again, see Ben's sig for that (it's free).

You should check your chlorine and pH daily, and adjust them as needed. Your ideal chlorine level is determined by your CYA level. Generally we suggest CYA between 30 and 50ppm. But with the K2006, you can measure the CYA level, so that test should be done weekly, as well as the total alkalinity test, and, you should check calcium hardness once in a while (like twice a summer) with a vinyl pool to make sure the level isn't too high.

Carl