Cooler fall weather but increased algae

[IneedHelp]

Our pool has been quite clear and free from algae this year, however with the cooler weather I have been having a little algae problem.

A chlorine shock clears it up quickly.

Why am I getting algae despite relatively good levels of chlorine - 2-3 according to my test kit.

Ian

[Larry]

Hi Ian
Could you give us some more information about the water?
Test your water or have it tested and post the following results:

Free chlorine
Total chlorine
pH
Total alkalinity
Hardness
Cyanuric acid (stabilizer)

Thanks
Larry

[IneedHelp]

Thanks for the reply

I have a simple test kit that measures chlorine and ph but I don't know how to measure the others. I called the guys who made my pool (and who gave me the test kit) but they said there was no need to worry about the other things you wrote.
I will try to get my water tested and give you the results then.

Ian

[IneedHelp]

So I got my water tested by a guy nearby who has a digital minilab for pool water testing and these are the results

Free chlorine 5,0
Total chlorine 5,0
pH 7,5
Total alkalinity 125
Hardness 270
Cyanuric acid (stabilizer) 185

So now I know that my problem is probably the stabilizer. I read a little and it said that it was from the chlorine. I checked the bucket I have and it says 56% dichlor (SDIC). I guess it's time to change my chlorine (and pool company).

What is the best plan of action for me?

Cheers
Ian

[chem geek]

The following are chemical facts that are independent of concentration of product or of pool size:

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added Cal-Hypo, it also increases Calcium Hardness (CH) by at least 7 ppm.

Obviously the guys who made your pool don't know a thing about pool water chemistry so I suggest you stop listening to them. To prevent green and black algae growth in pool water regardless of algae nutrient (phosphate, nitrate) level, the FC needs to be at least 7.5% of the CYA level. For 185 ppm CYA, that means the FC needs to be a minimum of around 14 ppm. So there's no surprise as to why you are getting algae growth.

Your plan of action is first to dilute your pool water with multiple partial drain/refill (or other technique to dilute the water such as continuous fill and drain) to lower the CYA level at least down to 80 ppm if not lower. Then to prevent the CYA buildup, you need to use chlorinating liquid or bleach. The downside is that you'll have to add chlorine every day or two unless you get an automation system. However, if you were using Dichlor, then you must have been adding chlorine frequently. I'm surprised that you weren't using Trichlor pucks/tabs in a floating dispenser or inline chlorinator as that is much more common.

[IneedHelp]

I've ended up draining my pool and a good thing too.
After a light acid wash I had the grout repaired in a couple of places and replaced the seals in the light niches.
I filled the pool with our well water and added about 2kg of the old chlorine. I also bought a bucket of "shock chlorine" which apparently doesn't have any stabilizer. It says 70% Calcium Hypochlorite on the label so I'm all set
Thank you for the great advice
Ian