No. Your FC should never fall below the minimum based on your CYA as noted on the FC/CYA Levels, not to mention never fall to zero. That, combined with a high pH level can contribute to calcium scale and scale-related staining. First thing I would do is add enough liquid chlorine to get your FC up to about 8 ppm and keep it there today if you can. Tonight start an Overnight Chlorine Loss Test to see if algae has started.
1 quart of chlorine, if you are using the 10%, adds 1.7 of chlorine to your water each time you do it. In Houston, you could burn through 2-4 ppm this time of year, so if you are adding it only every three days, you are not adding enough chlorine to the water at all.
While @Texas Splash is correct, it is not normal for a pool to fall that low, the honest answer is the result of adding such little chlorine every three days, it would be normal to see zero chlorine when you test. That is not the normal way to maintain proper sanitization, but the result really should be expected based on your normal routine.
right there is the next step. Chlorine of zero for multiple days, and maybe 1-2ppm for a few hours is great for an algae party. You don't want that in your pool, and if you want to get rid of that, and take control, listen to the great advice and you will have a trouble free pool in no time.
Tab feeders are a recipe for trouble when used as main source of chlorine addition - TFP methods advise using other sources for chlorine, either liquid bleach addition, or salt water chlorine generation.
That is reassuring. At this point I would recommend keeping your FC in the 8-9 range using liquid chlorine or regular bleach. Be sure there are no tabs in use for now. Keep the FC nice and strong in that target range and brush the pool surfaces well. Do not let the FC fall below 6 ppm. Let's see if the residue you were seeing previously gets eliminated and picked-up by the filter. If it continues to return or gets worse let us know.
A salt water chlorine generator!
Absolutely. I installed a Circupool model about 3 years ago. The link is in my signature, but that same holds true with many other models. You can do some online window shopping at Discount Salt Pools. Look for the Comparisons tab and be sure to look for a SWG that is rated for at least 2X the size f your pool. All pools retain salt, even those with liquid chlorine (salt by-product), so don't let that sway you.
