SWG stability

[intexpool]

I have 18' intex 5400g pool and use tfp test kit for any figures I use. I am currently using a newer model intex swg made for up to 10,000g I believe and is connected to an oversize Hayward cartridge filter. Typical chemistry PH 7.2-7.4, ALK 80-90ppm, CYA 60-70, FC 6-8ppm. Seems everything has run fine with SWG running 3hr day on average with filter running 6hr. Solar cover is used most of the time pool is not in use, and filter runs a few extra hours if pool is used. Seems after using the pool for a couple days straight with 2-4 young kids pool goes out of control as far as chlorine. Chlorine dropped to 5 which seems to be acceptable level, have been running SWG extra time and chlorine drops fast! Last night level was at 11 and dropped to 7 by morning. Second time this is happening, I get combined chlorine reading, but is under 1ppm, just like last time(usually at 0 any time I checked) The pool is not used often is this typical or what can cause it? Only thing out of the ordinary is different swimmers, 1/2 inch rain water, pool went a several days without vacuuming due to intex adapter failure...has only been 3 weeks since last time pool has been shocked and only a handful of uses. Doesn't make sense to me?

 

[needsajet]

Welcome to TFP! Good to have you here

Overnight chlorine loss is usually due to algae, but there's a specific way to test. Follow this test to the letter, and you'll know if there's algae floating in the water. When it starts, it's not visible to the naked eye, but this test will tell you.
Pool School - Perform the Overnight FC Loss Test (OCLT)

If you had the cover on a lot, and then had it off, combined chlorine might have built up and then burned off. Try leaving your cover off for part of the day or every other day to allow the sun to burn off CCs regularly.