Where does my chlorine go at night?

[Spoolspud]

Hello,
I have a small in-ground pool (about 2500 gal)that I've been taking care of for the past 6 months or so. I had been adding enough chlorine to keep it from turning green, testing the pH, and scrubbing algae off the sides every so often. I didn't realize there was an issue until about a month ago when I learned that its not normal for chlorine levels to drop drastically. I've been trying to SLAM the pool, first starting 2 weeks ago. I cleaned the filter, lowered the pH to 7.2, and started dumping about 1/3 to 1/2 a gallon of bleach or chlorinating liquid in 2-3 times a day. The kit I was using only measured as high at 5ppm chlorine, which I thought was fine. After about a week of this, nothing seemed to change, FC still dropped overnight, but there was very minimal CC.
Now, I got a new testing kit (the Taylor one with the FAS-DPD chlorine test and a CYA test). Now that I accurately know what my pool levels are, I'm even more confused.
So here's how yesterday and today went: at 7AM, about 12h after adding 2 cups of liquid chlorine, I tested FC and TC with my cheaper kit, FC is 0.5, TC seems about the same. I add a half gallon of liquid chlorine, let it circulate for about 30 minutes, and drop the floater in (trichlor tabs). The Pool gets direct sun from about 9AM to 4PM, and I'm in AZ so I need the stabilizer and more consistent chlorine dispensing that only tabs can provide.
Anyway, at 6PM I get home and use the cheap test kit: FC is about 1ppm, TC looks like 2ppm. And the floater was in there all day (ugh). I remove the floater and add about 2 cups of bleach. One hour later, my fancy Taylor kit arrives and I immediately measure everything I can: FC: 15ppm, CC: 4ppm, pH: 7.5, CYA: 30ish ppm. Since there is CC, I add 1/3 bag of potassium peroxymonosulfate shock.
After running the filter all night, I test in the morning (before the sun hits the pool): FC: 0.5? (almost no pink color), CC: 2ppm, pH:7.4

What should I be doing? Should I be adding even more bleach to my tiny pool? I was hoping I could rely on tabs for several days in a row this summer, as I'll be going on short trips and I don't want to ask someone to come over twice a day to add liquid. Why can't tabs keep my FC level above 1.0?

If you've read this far, I appreciate you. I hope someone has some kind of advice

 

[Major_in_MS]

The Pool gets direct sun from about 9AM to 4PM, and I'm in AZ so I need the stabilizer and more consistent chlorine dispensing that only tabs can provide.

The relationship between CYA (stabilizer) and Free Chlorine (FC) requires that as your CYA levels rise, your FC levels must also rise to have the same sanitation effect. The problem with using the trichlor tabs as your primary chlorine source is the fact that the chlorine part will be consumed by sanitation and sunlight, but the CYA part will only ever be lost by removing water from the pool and replacing it. This causes CYA levels to continuously rise, causing increasing chlorine demands, until the chlorine required for proper sanitation reaches an unattainable level, and then a large water replacement in needed to bring CYA levels back into line.

For this reason, we use liquid chlorine, or salt water chlorine generator cells (SWCG) as our chlorine source to prevent the constant buildup of CYA.

CYA: 30ish ppm

The CYA test requires sunlight to read correctly, the light refraction index of the water changes causing the black dot to disappear sooner at higher levels.
Testing conditions can give false readings. See CYA Testing - Further Reading for more details.

I'm leaning towards thinking that your CYA level is actually much higher than this.

 

[Major_in_MS]

FC: 15ppm, CC: 4ppm, pH: 7.5, CYA: 30ish ppm

Don't bother to test pH when FC is >10 because the results are invalid.

After running the filter all night, I test in the morning (before the sun hits the pool): FC: 0.5? (almost no pink color), CC: 2ppm, pH:7.4

You need to bring your pH to 7.2 and then begin following the SLAM procedure until completion criteria is met.

 

[Major_in_MS]

Why can't tabs keep my FC level above 1.0?

The tabs are not releasing chlorine fast enough to overcome the algae growth. The only way to kill off all your algae is to SLAM, bring to Shock Level And Maintain. The maintain part of that requires frequent testing, no more than hourly, and addition of more chlorine to bring it back up to shock level.

 

[CrystalRiver]

Your chlorine is disappearing overnight (and during the day) because you have algae. The chlorine is doing its job. Once you get rid of the algae, your chlorine consumption (both day and night) will drop considerably.

 

[Texas Splash]

Spud, I see you not only have a small pool that would qualify for using the seasonal pools system, but it's inground. So unlike an AGP, it's not as easy as pulling a plug to drain. But rather than go through an entire slam process in such a small pool, have you considered using a small sump to drain most of it? I would think it should be possible to do so safely with your set up and hard ground. Trying to slam such a small pool can be challenging.

 

[Major_in_MS]

Spud, I see you not only have a small pool that would qualify for using the seasonal pools system

Yep, I almost included that link but didn't because it is an inground pool. But this is definitely a good option, while empty a good scrub of everything with a nylon brush and some bottle brushes for the fittings with a solution of bleach/water (1 cup to 5 gallons) would be a great decision.

 

[Spoolspud]

Yep, I almost included that link but didn't because it is an inground pool. But this is definitely a good option, while empty a good scrub of everything with a nylon brush and some bottle brushes for the fittings with a solution of bleach/water (1 cup to 5 gallons) would be a great decision.

This is a good idea, thanks. It has a heater attached, so we keep it open in the winter for the occasional hot tub experience. At the end of the year I'll definitely look into this!

 

[Spoolspud]

Spud, I see you not only have a small pool that would qualify for using the seasonal pools system, but it's inground. So unlike an AGP, it's not as easy as pulling a plug to drain. But rather than go through an entire slam process in such a small pool, have you considered using a small sump to drain most of it? I would think it should be possible to do so safely with your set up and hard ground. Trying to slam such a small pool can be challenging.

I'd been avoiding draining it because it feels like such a waste of water! But, I'm realizing that partially draining it is getting me nowhere and is just as wasteful... Me and the spool now have major weekend plans. Thank you!