Very sudden drop in CH levels?

[DUCK01]

I’m trying to figure out if or how my CH levels dropped 100ppm in 2 days. Since May 1st, I’ve been 450, 450, 450, and most recently 475 on May 29th. Im in the process of checking chemistry now and, after 3 tests from 3 different locations in the pool, I’m consistently 375ppm. Same reagents and no unexpected swings on other levels. I did add about an inch of water Sunday after I tested my levels (not sure how that equates to gallons, unfortunately), with plans to retest today once the heavy weekend use was done. I’ve added the same amount of water in the past and haven’t noticed any big swings in CH levels, though. I’m always consistently in the mid 400s.

Other levels from today, using my usual TFPro test kit.
CL 8
CH 375
TA 80
PH 8.2
CYA 50

 

[AK-]

That would be equivalent to a 20% dilution...
Since you already said nothing else changed and that you manually top off the water then I guess a leak and constant refilling can be ruled out.
Other options are bad reagents (how old are your reagents?) and testing error.

CH don't just go away...

 

[DUCK01]

My reagents are from last Fall. This is my first Summer with the pool (new build completed early last Fall), but I’ve been adding about an inch of water a week (slightly more or less depending on user load and how much water the kids splash out).

I’ve noticed nothing out of the ordinary and almost didn’t run a full test because I just checked a couple of days ago. Only reason I decided to check this evening is because we’re heading out of town until next week and I wanted the confidence that all is well. So much for that.

Testing is the same as always - I follow the cheat sheet that came with my test kit and don’t try to go by memory.

I’m hesitant to add Hardness Plus because I’m afraid something else is off and I’ll push my CH levels way too high.

 

[C3Cl3N3O3]

I'd revisit your test procedures and equipment; those results seem suspicious.

Water Testing Instructions - Further Reading

www.troublefreepool.com

 

[DUCK01]

I just tested my tap water (what I use to fill) and it’s almost no calcium. I added one drop of R0012 and the sample turned instantly blue. I basically diluted the calcium in our pool water when I filled it.

Our water supplier isn’t the best and we have wild swings in water quality (from overly sulfur smell one week to heavy chlorine smell the next), so no telling of what I just tested is normal or not. I’ll add about half of the Hardness Plus suggested to get my levels back up and will recheck in the morning. Based on that, I’ll have my answer.