TABS contain cyanuric acid. TABS are designed to raise your Total Chlorine to proper levels in direct sun light as they float on top of any stabilizer level in the water. They are NOT designed to replace your Free Chlorine levels when needed.This means you are swimming in water that takes 30 min to kill bacteria, all germs washed off of the swimmers bodies including yours are alive for approximately thirty minutes after making contact with your TAB's chlorine.
This applies to algae blooms as well including black algae. Algae can outgrow when your combined chlorine is .06 ppm higher than your free chlorine. TAB's include high levels of stabilizer on purpose as they are in direct UV sunlight. That super locked down chlorine takes care of the bird poo or dog slobber etc and free your Free Chlorine to kill bacteria and algae in UNDER A SECOND. Take a look at the Test Strip chart below. Notice the FREE CHLORINE & TOTAL CHLORINE DEFINED. Just subtract the Free from The Total and you will see your Combined Chlorine, if it is 1ppm you really need to shock the pool (Raise Chlorine 10 PPM for every PPM Of Combined)
Keep Free Chlorine Level Up Weekly With Liquid Chlorine or Powdered Cal Hypo small additions only and you will not need to ever shock. Easy Rule To Follow. Never let the Free Chlorine Slip Below 2 ppm for two days in a row and you will never need to shock. Use the dosage charts for Liquid to see what you would need for your size pool.
Also encourage you to read the Chlorine Blog for more info on Free Vs. Total Chlorine.
*Warning, using TABS alone as the main chlorine source increases CYA levels and this requires you to use double and triple the amount of chlorine to kill. Especially cheap box store tabs that are paste instead of using an expensive machine press to form pure chemical paste tabs from overseas uses glue and fillers. They melt fast and shed CYA faster!