Black algae, low FC with a SWG

[duraleigh]

I meant how high do I need to bring the FC to kill the ALGAE.

With a CYA of 40, it will be necessary to bring your FC up to around 20 to combat black algae. The algaecide will not help you.

Here is an important thing.....Read "How to Shock Your Pool up in Pool School. You will find that it is a process not a one time dosage. If you are serious about getting rid of black algae, you will have to MAINTAIN your FC at the least around 20ppm. This requires constant testing and additions of liquid chlorine to bring FC back up to 20 each time it lowers.

Secondly, you will need to brush the spots with a stainless steel brush daily until the spots are gone. Last, you will need to maintain your FC up around 6-10ppm for at least a few weeks after the black algae is gone and I would suggest keeping it there all summer

Black algae is VERY troublesome, but if you want to get rid of it, you won't be able to do it without a serious, daily program.

 

[Hotjez]

Thanks. Added acid today will scrub and shock tomorrow!!

 

[Hotjez]

Hi everyone! Update. I went down on scuba armed with a wire brush and 3 inch tablet ..scrubbed off 389 ba spots (yes I counted) and then rubbed with the tricolor tablet. I then added 5 gallon of liquid cl to my 15k pool. Two days later my pool guy vacuumed and added 1/2 g acid and 1.5 g algaecide. I am hoping for the best!

 

[JasonLion]

If your CYA level is still around 40, the you should raise FC up to around 16 to kill regular algae. However, that won't work reliably on black algae. For black algae you need to brush frequently, or rub the algae with a trichlor tablet.

 

[Hotjez]

Jason, so I did the calculation...for a 15k pool I calculated that adding 4 gallons of cl would raise FC to about 30. I added 5 to be safe. But my Taylor tester only goes to 5 so how to you guys assess FC levels higher than that? Do you think I need to scrub again, even without any visible spots?

 

[JasonLion]

It would be a huge help if you got a FAS-DPD chlorine test, which can measure FC levels up to 50. Lacking that, you can get very approximate levels using your current kit and dilution as long as you have some chlorine free water (distilled water?). Mix one part of pool water with two equal parts of chlorine free water, do the test on that, and multiply the result by three.