Calcium Chloride substitute

[mikeinbkk]

I have an 11,000 gallon in-ground fiberglass pool.
I've had relatively low calcium/hardness since day 1 (the pool is a year old). I'm getting some buildup on the pool walls and a bit on the floor. I use a salt water chlorinator, cartridge filter and turn over the water 3.5 times a day over 9 hours of filter runtime

My latest numbers:
Ph 7.2 (i keep it between 7.0-7.4 measured with a Hanna ph meter)
TA - 80-100 (test strip - originally low but got it setup pretty quickly)
salt (3500ppm Emaux salinity (not TDS meter) meter. I keep it above 3000)
TC - 2.75 (Ponpe digital meter/test strip and sample well sort of a titration-based system I think)
FC - 2.2 (same system as TC)
Hardness 120ppm (Hanna titration type kit)
CYA - added cyanuric acid about 6 months ago to 40-50ppm (test strip measurement) and have basically ignored it since but the last time I used a test strip, it was still about the same (I hate test strips. I'm a meter kind of guy)

My tap water is soft...typically 40ppm as measured with the titration kit...so top-ups are very low calcium. I live in the tropics (Thailand) so I top up water about an inch or so 1-2x/week (1inch = about 200gals). I'm thinking about a simple floating cover to reduce evaporation as I'm losing maybe 10% of my water a month - it seems like a lot, but I don't know what I'm doing - so maybe that's fine. As a final note on the subject...there is no winterization here - the pool is up 365 days a year

I've kept ph and chlorine pretty stable since day 1, though I had some bounce early on when I was learning to manage pool chemistry.

when I first noticed the scale buildup, I called the local pool supply guys who also build concrete pools. I got no real practical advice - just a rant about how fiberglass pool guys were selling substandard pools that would accumulate calcium scale and crack from bad installation - clearly some sort of case of bad blood.

What I need is some direction on how to manage/control this buildup. I've been using a chelation treatment every 10 days according to instructions with spotty success but nothing else unless I need to balance ph (34% muriatic acid and maybe 1 time some soda ash when my ph went low). I've never shocked the pool but water is crystal clear. My chlorinator does not have a super chlorinate function and I run it at 80-90% to get the chlorine numbers I have.

Any advice would be much appreciated

[Teapot1]

You are following a regime with TA and hardness that mirrors the industry instructions for a pool, that is for a concrete/tiled/plaster type pool. As you have a plastic pool you can do something different, more akin to plastic and vinyl lined pools as you wont degrade the finish as you could with concrete etc.
Forget raising TA, if your water supply is at 40ppm great., wish mine was (180 -240ish) here. You can safely run with low calcium hardness and low TA but I would increase the pH a little to compensate for the slightly aggressive water according to langlier saturation data.

Years back I was at a customers pool in France, I was trained by the industry to adjust all chem water levels to the figures. His water is super soft, TA at 27ppm hardness was around 60 from memory. I didnt sleep wondering where I could buy calcium chloride and bicarbonate of soda in bulk! I woke up after little sleep and my engineering background had taken over. Why if his TA and hardness were way off was his pool fine??? Also had been fine for a long time (I was there for a different reason) because with a plastic or vinyl pool you do not need the same level of TA or hardness but the industry hasn't recognised this (great for sales! Maybe a reason).
I went home and lowered my TA to 40ppm didn't worry about the hardness and have been running it that way for 12 years. What you will notice is you need only a tiny amount of acid to bring the pH down as the buffer (TA bicarbonate of soda) is much lower. This also means the pH will not rise as much, if at all. I haven't added pH minus in 2 years! This has been duplicated many times with my customers all reporting improvements and barely adding pH minus.
The chelation will cause calcium drop out and just hoover it up but you should see an improvement over time. There is no need to keep the pH down so low as its easier to allow it to find a better level around 7.6-7.8 this will not diminish the chlorine ability as you have CYA stabiliser in the water. It is true that the higher pH does diminish chlorines killing ability but that is when CYA is not present like indoor pools. That should save you a lot in acid. Do try to get a CYA tester a disappearing dot type if you can strips are hopeless and CYA is the most important chem in your pool as it runs the chlorine level and buffers pH down from TA.