Welcome! First, I think your estimate is 'way too high. a rectangle 18x36x 4' deep is 2592 cubic feet. At 7.48 gallons per CF, you'd have 19388 gallons. Obviously, curves and angles make calculation more complicated, but if you know the cubic feet, multiply by 7.48.
Do you have a reason to think your sand filter is either defective or too small? It may be, and going to a bigger sand filter will generally give you better performance.
We call them "Guess Strips" because they are almost impossible to read accurately. We recommend the Taylor K-2006 instead.
http://www.poolforum.com/pf2/showthread.php/14994
What are you shocking with? If you use Dichlor or Trichlor and your stabilizer level is high you'll just make the problem worse.
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