Pool & spa still eating chlorine like there's no tomorrow

[Denniswiseman]

If you are happy with your CYA of 35 you would need a minimum of 3 with a target of 6-7 with a shock of 14 as indicated in Chlorine / CYA Chart although you could do with a larger CYA of 50
To eliminate CC you would need to Slam (Shock Level and Maintain) which is a continous process and needs testing regularly during the day. That is put enough liquid chlorine to reach shock level and test, then test again in a hour to see how it's moved
How old is the bleach as it doesn't hold full strength for to long and could virtualy be salt water
Forget about phoshate remover unless you have algae as it's algae food
Could you possibly have algae in hidden areas

[Teapot]

It could be nacent algae just waiting to pounce as soon as the chlorine lowers to 0.
It could also be naff 9.6% chlorine. That's the strenght when they make it but poor storage in warm weather and time can really break it down. I have tested some and calculated it to be around 4% chlorine hence you would need around 2.5 times as much. It's only €12.90 for 20 litres so go buy some more but maybe from a different Brico or look at the manufacture date, fresh is best!
You should test for phosphate if you are around farms like I am, it's not just algae food it's like caffine and chocolate brownie to algae. Around August we get an algae bloom so shock dose and hold that level for a day or two just to be sure. Keep your chlorine on the high side.

[witters]

Thanks, guys
The chlorine was manufactured on 20 April with a use by date of 20 October. It's kept in the barn which is always relatively cool (below 25 degrees). I will get fresh supplies and persevere. My surprise was when I used shock levels of granular choc (when I ran out of bleach) this also disappeared very rapidly. I suppose even the granular stuff can lose its strength, but the tub I've got is supposed to be good until October 2021.
Nascent, unseen algae seems to be the only logical explanation so I will continue to nuke the pool and see what happens. What I haven't done so far is measure every hour or so to check the rate of decline so I will do this and hope that I can maintain some sort of decent SLAM routine.
Thanks again

[Teapot]

I had a guy in France that somehow got a fair amount of what seemed to be ammonia possibly from kids peeing. That took a lot of chlorine to iradicate, lots more!

[Eddles]

Update:

I received the Taylor K2006-C test kit the day I partially drained the pool, so I decided to do a test while the pool was draining:

FC: 4.6 (interesting, as my other test kits all say 0.9!)
TC: 5.6
pH: 7.4 (other test kits agrees)
TA: 325
CH: 350
CYA: way way way off the scale - literally only 5 drops was enough to hide the black dot.

The pool is currently filling up and will do another test tomorrow.

After finishing testing the pool, I decided to test the hot tub:

FC: 13
TC: 16
pH: <7.0 (need 8 drops to raise up to 7.4)
CYA: way way way off the scale
TA: didn't bother to test
CH: didn't bother to test

After I did the CYA test, I realised the water was a write off, so I drained the hot tub fully and refilled it.

An hour after refilling with fresh tap water, despite the water still being a bit cold (~20°C), I did the test again:

FC: 0.0
TC: didn't bother
pH: 7.8
TA: 190
CH: 190
CYA: 0

Much better, but TA a tad high. So I put in 14g dry acid (1200 litres spa) and hopefully TA and pH will drop a bit when I test again in the morning. I also added in a 20g stabilised chlorine tablet to add in some CYA. I won't add in more until I do another CYA test. Fingers crossed!

Question: when I start to use liquid chlorine for the hot tub, how much do I need to put in?

Many thanks again for your help!

[Denniswiseman]

Congratulations on getting a decent test kit, you can now control your water better
Use Pool Maths to work out how to raise or lower the different numbers
I find 300ml of 14.5% will raise 45,000 litres 1ppm
Stick with the Chlorine/Bleach once you have your CYA at the desired level

[Eddles]

OK, so the pool has filled up, 60% of the water was changed, and I've done another series of tests & chemicals:

08.08.19 - 10:00
FC: 1.2
pH: 7.4
CH: still off the scale, but not as bad as before - needed 10-11 drops to hide the black dot.

08.08.19 - 22:00
FC: 0.5
pH: 7.6 (but I struggled to see this one)
TA: 250
After the above test, I added in 2 litres of 15% bleach and 975g of dry acid:

09.08.19 - 08:30
FC: 2.5
pH: 7.4

So, after the fill-up, TA has gone down from 325 to 250, and CyA has gone down from a ridiculous amount to a silly amount, which is an improvement anyway. The pool seems better, my eyes stung a bit after a swim today. I'm aware CyA is still excessively high but chlorine seems to be working better as it's lower than before. I will lower the pH to 7.0 and aerate the pool, to try to lower the TA further. Hot tub water chemical levels are lovely.

I'm much happier with the new water, but I'd like your thoughts please?

Many thanks!

[Denniswiseman]

You have CH for CYA, you really need to concentrate on getting your CYA down to a more acceptable amount Chlorine / CYA Chart
Your eyes stinging is because you have chloramines (used up chlorine)
You don't mention your combined chlorine

[Teapot]

Seconded!