Black algae

[max]

Today I noticed about a dozen black dots on the pool, the largest about half the size of a fingernail but most were about a quarter the size of a fingernail. I'm not sure how long they've been there - I usually swim at night, the water itself is clear and the spots were blending in very nicely with the black pebbles! I gave them a good scrub which got rid of about 90% of the bulk leaving a thin green smudge. I then shocked the pool and intend to keep the levels up for about a week (over 20ppm for CYA 60ppm). I'll keep scrubbing though I'll have a bit of trouble finding them now.

Is there anything else I should be doing?

Thanks,
Max.

[max]

PS PH 7.6, TA 150ppm, CH 138.

[chem geek]

You should probably first determine if these are truly black algae or if they are metal stains. Given that you could scrape them off somewhat, they do sound organic. If you take a trichlor puck and place it over one of these spots, then if it fades, it's organic and if it tends to grow (slowly) or come back, then it's likely black algae. Long-term shocking will kill it, but the key is to frequently scrape the surface to expose the interior to high chlorine levels. Black algae forms a chlorine-resistant slime layer so you really have to constantly remove that, even several times a day (at least at first). It's a hell of a battle which is why it's much better to avoid black algae in the first place by maintaining sufficient chlorine levels relative to CYA -- black algae grows slowly so it's a problem with an inadequately maintained pool over months, not days.

[max]

I tried the trichlor puck on the few I could find - they were starting to look slightly brown and the puck finished them off. I don't think scrubbing will get any more without damaging the surface because the bit that remains is between the pebbles. I'd scrubbed it down from about 3mm thick to maybe 0.2mm which is below the level of the pebbles which stick out about 1mm. I'm using a decent good old fashioned wooden handle scrub brush which has made the pool surface spotlessly clean, taking care not to overdo it. The pool *looks* incredibly clean anyway!

In terms of past maintenance, it was my pool shop which frowned upon FC levels above 3ppm, regardless of CYA levels, therefore I had a SWG setting which occasionally resulted in lows of 1ppm. I now realize that 3ppm is the minimum required for 60ppm CYA and I will ensure that is the case. I will ignore my pool shop in future!

Thanks,
Max.

[max]

I've been reading more about black algae and trying to get hold of close up pictures (without being microscopic) but most are from a distance. Can somebody point me to a picture which shows black algae from no more than a foot away? The pictures I saw didn't remind me of what I picked and scrubbed off the pool which was more like little domes. If you imagine a small peanut split in half and sitting flat side down - that was the biggest one. It had quite a bit of thickness for its size. You could pick at it and the bulk would come off in fairly solid pieces that you could pick up if you wanted. It was not slimy at all. After scrubbing what remained for about 5 seconds it was difficult to see anything left of it and I've got good eyesight. Does that sound like black algae or might I have some other strange thing?

Max.

[chem geek]

Look at this link which will first return a web page saying "Forbidden", but then click on the link in the URL entry field in the browser and it will come up. This example is a crescent, but others are more solidly filled. Note how the shape is somewhat irregular on the edges and that it is a pretty solid and dark black. It does have some height to it, but not too much.

Of course, in your pebble pool, if the surface is uneven then the black algae could grow with varying thickness as well. Does what you have look at all like the link?

[max]

That's a good photo - thanks. My black domes looked nothing like that. They were very well formed jet black domes that were so perfect they looked like they could have been manufactured black gemstones. Not rough or randomly formed at all which is why they blended in so nicely.

Perhaps this is something not found or common in North America - maybe I should make some local enquiries.

The surface of the pebble pool doesn't vary much - less than 1mm I'd say (0.05 inches). The pebbles are very small with varying colours to give a speckled look. The domes were up to about 0.2 inches above the pool surface - the largest could be picked off with a fingernail and a good scrubbing brush got most of what remained. I didn't want to keep using my fingernail so I found a hard bit of plastic to pick with (not to scrub with).

Max.

[max]

My local pool store were of the opinion that it was blackspot algae which I assume is black algae. One thing I like about my local pool shop is that they don't try to sell me anything I don't need (like algaecide), just what I ask for which was a good scrubbing attachment with much stronger bristles than my old one.

I'll continue to scrub several times a day over the next week while keeping FC at a minimum 20ppm. I can't measure FC past 10ppm accurately but I figure if it looks way off the scale on its own and looks at least 10ppm when mixed half and half with tap water, it must be up there somewhere. Not accurate enough to measure FC loss but it'll have to do. My pool shop was horrified with this level!

What gets me is that I've always been so obsessive about my pool levels, it's just that I was led to believe 1ppm to 3ppm FC was a good range (I was told this again today) without any consideration to CYA. I won't fall for it again... EVER!

Max.

[max]

That was 10 months ago and things have been fine since then. In fact I don't think I had a nasty form of algae after all. When I described it some months later to a pool person I get in for occasional repairs, he seemed to think it was a common algae in our city and not particularly difficult to get rid of which is what I found. I respect his opinion because he's been servicing pools for a long time and always has good advice.

Oh well, the pool sure looked clean after all that scrubbing!

Max.