Black Algae - black pebbles

[max]

I discovered about a dozen black dots/domes on the weekend around the quarter the size of a fingernail which were blending in very nicely with my black pebbles. About half were on my top step. Because I nearly always swim at night, I hadn't noticed them. I scrubbed all the ones I could find down to a dark green smear and shocked the pool so FC is 20ppm (CYA is 60ppm). I have since been scrubbing every inch of the pool several times a day with a new (decent) scrubbing brush with good strong bristles. Visibly things are looking brilliant but having read the other threads I know not to be complacent - I continue to scrub the whole pool several times a day and keep the FC at 20ppm. I haven't seen any dramatic falls in FC though my measurements at that level aren't accurate - I'm diluting pool water half and half with tap water and reading 10ppm (if I use straight pool water it is way off the scale). I realize this is not a good way to measure FC but I figure the FC must be up there somewhere. pH was 7.6 prior to shocking - I added some acid on advice from my pool store and it is probably around 7.0 - I won't be adding any more until I lower the FC and get a new measurement. TA was 150 and CH 138.

What I'm wondering is... now that everything visibly looks fine, how much longer should I maintain shock level and scrubbing to ensure it's all dead? I propose the following plan:

20ppm FC for another week scrubbing several times a day
8ppm FC for a week after that scrubbing once a day
5ppm FC thereafter (minimum 3ppm or 5% of CYA), scrubbing weekly

I've always been quite obsessive about my pool but I was led to believe that 1ppm to 3ppm was a good range of FC regardless of CYA. I've since been enlightened! My FC was usually around 3ppm but it did drop to 1ppm sometimes which is how I must have got the black algae. My useless old scrubbing brush probably didn't help matters - I may as well not have been scrubbing at all.

Anyway, does the plan look good enough to be rid of it for good or should I be attacking it for longer at higher levels to make sure?

Thanks,
Max.

 

[max]

I should point out that only about 10% of the pebbles are black - the other 90% are light coloured with the black ones creating a speckled look. The pebbles are no bigger than a match head so black algae bigger than a match head is more obvious. It's difficult though because two black pebbles adjacent to each make a false alarm so I have to look very carefully at close range in full daylight. I have good eyesight so I'm hoping I've found it all. I've reluctantly been in the pool with old swim gear, clear goggles and a lot of breath holding while I closely examine the whole surface - so far I've survived. I still could easily miss one if it's as big as a match head which is why I scrub every last bit of the surface multiple times (from outside the pool). I keep telling myself "this bit I'm scrubbing now is the last of the live black algae which I'm about to expose to the chlorine" because that may well be the case!

It all seems so open ended though - I wish a bell would ring when it's all dead!

Max.

 

[JasonLion]

Welcome to TFP!

I am sorry you have black algae, it can be very annoying.

Your plan sounds reasonable. It can indeed be tricky to know when you have gotten it all. Hopefully you will have it all cleaned up soon.

 

[max]

Thanks.

I've discovered what might be the source of my black algae. At the very top of the pool there is a gap underneath a few of the horizontal terracotta tiles that border my pool. The tiles protrude further than the plaster so it is not obvious unless you run your fingers underneath. When I first moved in 15 months ago, the pool was overflowing and the existing SWG chlorinator (probably 8 years old, the same as the pool) was on its last legs and struggling to produce sufficient chlorine at maximum setting. At the time of the sale, nobody was living there. It had been nicely presented for the sale but by the time I moved in a month later the pool was well and truly overflowing with recent rain. I've kept the water level at normal skimmer basket range ever since. I'm now wondering if the black algae's been lurking above the water line there, receiving splashes every night of chlorine at normal levels to keep it happy but practically none of the shocking I've done, not even splashes, as I'm not swimming then.

I've rubbed a trichlor tablet on it and I assume it would be a good idea to add water to the pool to bring it close to overflowing so that the very top of the pool is being shocked along with the rest of the pool. Does that sound sensible?

Max.

 

[max]

PS I contacted a local pool maintenance guy who showed me the ropes when I was a newbie and seems to have a lot of practical experience. For that area above the water line, he suggested spraying some chlorine so that it gets right in (the trichlor tablet can only reach so far) and suggested if I'm concerned that I may as well spray right around the top of the pool with it. I'll do that once a day while I'm shocking so that the whole wall gets a good treatment (and of course won't forget about the scrubbing!)

 

[max]

PPS I should point out the chlorine I sprayed is not the full liquid chlorine concentration - I mixed about a level teaspoon of granular chlorine to a pint of water and sprayed it right in there. It hasn't caused any noticeable bleaching as yet.

I'm still a little tempted to raise the water level so the whole wall is being shocked... or is the top part of the wall best left dry?

Max.