How much chlorine do I need to kill black algae, given these paramenters?

[MrLeadFoot]

This is my first post, so please forgive me if I leave anything out, or worse, have posted this in the wrong section! That said, I do not know if the information about my pool will automatically be appended to this post, so these are my pool stats:

25 year old plaster in-ground residential pool near Sacramento, California, approximately 26,200 gallons, NON-saltwater chlorine generating

I've been doing the typical naive pool owner thing for years: tabs in floater(s), and shock with pre-packaged "shock", based on readings from a cheap OTO test kit. This worked for a few years, then gradually became ineffective. Eventually, black algae started growing on certain parts of the walls and stairs during the warm weather. In the winter, the algae would die, but once the weather warmed up again, it would come back, and flourish in the heat of the 100 degree days of summer. I began adding copper algaecide every year which, again, worked for a few years, and now seems not to have any effect on the algae. The water, however, has always been sparkling, crystal clear, and I have never had an algae bloom.

Fast-foward many years to yesterday when, after months of researching and FINALLY understanding the detrimental effects of high CYA levels, I tested with a good Taylor kit.

FC 1.5
CC .5 (not sure it's even possible for combined chlorine to be lower than free chlorine, but that's what the test showed)
CYA 100
PH 7.4
TA 70
CH 340

Now, I understand that the only real way to lower CYA is to exchange via drain and refill, but where I live, it is almost unfeasible due to the high cost of water. For example, my water bill is $100+ a month just to barely keep grass and plants alive on my tiny 1/3 acre parcel. If I water enough to allow plants to thrive, the bill hits $200. I can't even imagine how much it would cost to refill my pool!

We get heavy rains in the winter, to the point that it looks like my pool is going to overflow, and I'm thinking that one reason the algae goes away then is because the natural dilution of rain water drops my CYA levels and the chlorine becomes effective. Of course, then I've been sabotaging myself by adding CYA-comprised tabs and shock again, every summer.

So, I'm hoping there's a way to be able to get chlorine levels in my 100ppm CYA pool high enough to kill off the algae and get me through the rest of the season, after which I hope mother nature can help lower my CYA levels to something more appropriate. Of course, I will immediately be stopping the tabs and shock-in-a-bag stuff the pool stores sell.

If what I propose is possible, can someone please tell me how much chlorine I need to add to accomplish what I want? Any other suggestions, recommendations, procedures, etc. are also most welcome.

Incidentally, I performed the above tests with the Taylor kit simultaneously with my OTO kit, just to see how accurate the OTO kits is in comparison, and I was surprised to see it actually measures up to the Taylor DPD and PH tests. Of course, the OTO kit doesn't go high enough on the chlorine side to be worth anything in my situation, nor does it perform any other tests, but at least I know that it is fairly accurate, and can used to at least test for the presence of chlorine, and test PH, in a pinch.

I anxiously await your replies, and thank you in advance!

[Denniswiseman]

With a CYA of 100 you need a shock level of 39 to Slam (Shock Level and Maintain) and a target level of 11-13 and a minimum of 8
Use these common products to balance your pool
Liquid chlorine (sodium hypochlorite or plain bleach)
Muriatic acid (hydrochloric acid) to lower pH and TA
Bicarbonate of soda to raise TA
Aeration will raise pH only
Soda ash will raise pH and TA
When you are finished swimming for the summer, drain half the water and let the winter rains fill it

[MrLeadFoot]

Thanks you for the reply. If I drain it that low, I can't run the pump. Is it ok to leave it like that for weeks at a time with no agitation, and not exprience stagnant water problems, or other issues? I thought there was also some issue with leaving the plaster exposed, or something along those lines. Sorry for the stupid questions.

[Denniswiseman]

Leaving the pump off doesn't cause a problem as a lot of people winterize any way
Leaving the plaster exposed for a considerable time isn't desirable so it may be an idea to do a partial drain and fill with rain often. Bearing in mind the cost of all the extra chlorine itmay be as well to just pay the extra cost of fresh water
CYA does seem to dissipate slowly over time

[Teapot]

Just to be clear, black algae is a pool industry misnomer. It's actually a tough near chlorine resistant cyanobacteria. You need to scuff off the tough outer shell and if possible get some chlorine dioxide onto it.
There are some enzymes that will eat your stabiliser but chlorine needs to be zero, or you'll kill the enzyme. Mixed results reported but in your situation nothing to loose.
I have also successfully oxidised the stabiliser with high dose and sustained high dose chlorine and I mean high dose! That could also assist your eradication of the cyanobacteria, that is a tough process on a plaster/rough finish pool where it can get a good grip.

[MrLeadFoot]

I do understand that black algae is cyanobacteria. I get that in my fish tanks once in a while. Interestingly, though, in fish tanks it peels up off of things really easily.

Can you provide more details on how you oxidized the stabilizer?

[MrLeadFoot]

Thanks for the reply. Just to be clear, was that a typo, or did you really mean that CYA dissipates slowly over time?

Leaving the pump off doesn't cause a problem as a lot of people winterize any way
Leaving the plaster exposed for a considerable time isn't desirable so it may be an idea to do a partial drain and fill with rain often. Bearing in mind the cost of all the extra chlorine itmay be as well to just pay the extra cost of fresh water
CYA does seem to dissipate slowly over time

[Teapot]

CYA does dissipate over time due to dilution by rain, top up water and slow oxidisation by chlorine. There have also been cases during winter and zero chlorine that a naturally occurring bacteria have eaten the CYA. That same bacteria also seem to leave an ammonia waste byproduct behind that requires a very large and sustained chlorine demand.

On the pool in question we could not empty as the water discharge was down the highway. We raised the chlorine level to also counter the gallons of anti-algae the owner had put in, it was a foaming mess when I arrived.
Raised chlorine level to around 100-120ppm with un-stabilised liquid chlorine and maintained it for nearly a week. 200ppm of CYA was reduced to around 30ppm.

Not sure if your copper level is low but on a plaster pool I would never add copper products. Using a very high level of chlorine could temporarily raise the pH above 8.3 and you could get the chlorine plating out and staining.

OTO whilst as you have found is reasonably accurate it's actually measuring total chlorine not free chlorine.