Possible black algae?

[nrs]

Hello, first post TFP lurker here. The pool described in my sig came into my possession about two months ago. The previous owner had a pool service, but ran tablets and CYA level was at ~375 before I had the pool RO'd on October 8th. The pool has had some problems including very high pump pressure (cleaning the filter fixed that), extensive leaf stains (vitamin C tablets did the trick), yellow/brown dust all around the drain (after RO and shock, all gone now), still working on problems with the heating and solar.

Just before the RO I wasn't good about keeping the FC up due to being both new at this and constantly on travel- I was trusting the tablets left in the chlorinator to do their job. Sometime when I was away or wasn't paying attention the chlorinator ran out of tablets, and these little black spots showed up in a small patch around 4" across, about midway between the edge of the pool and the main drain. The test kit I had at the time showed 0 ppm FC so I dumped 4 gal of chlorine in and scrubbed the spots, but no luck.

[attachment=0:3nqbp066]IMG-20131103-00314.jpg[/attachment:3nqbp066]

Fast forward it's been about a month. After the RO I've been keeping the FC high, up to 20ppm when I initially add chlorine and no lower than 10ppm, to try to help fight the spots which I can only scrub when I'm home about once a week. I can't say I'm passing OCLT as the pool seems to consume about 0.5-1ppm per night and the spots are still there. They haven't shrunk but they haven't spread either, the patch looks exactly the same as when I first noticed it.

Current numbers
FC 15
CC 0.5
pH 7.5
TA 120
CH 250
CYA 80

If the spots are indeed black algae do I just need to be scrubbing more often? The rest of the pool looks great. Admittedly I haven't explored the light niches yet to see if there's anything hiding in there.

 

[zea3]

Scrub the spots well with a wire pool brush, then scrub the spots with a trichlor tablet. Follow the slam process until your pool passes the overnight test.

 

[BoDarville]

Welcome to TFP!

If the spots are indeed black algae do I just need to be scrubbing more often?

Yes, brushing is the key to removing black algae. For black algae, this is as important as maintaining the elevated chlorine levels as outlined in the SLAM Process. Black algae forms a strong protective film over itself for protection. You need to break down this film so that the chlorine can get at the algae. You need to brush these spots at least twice a day.

Admittedly I haven't explored the light niches yet to see if there's anything hiding in there.

You will want to do that. See this post: http://www.troublefreepool.com/think-i-may-have-found-my-fc-demand-issue-t65957.html

 

[duraleigh]

Welcome to the forum.

extensive leaf stains (vitamin C tablets did the trick)

There's a lot to learn here and I don't want to pile on but if vitamin C removed the stains, they were iron stains rather than leaf stains. Leaf stains are organic and are removed by chlorine.

 

[nrs]

if vitamin C removed the stains, they were iron stains rather than leaf stains. Leaf stains are organic and are removed by chlorine.

Ah, thanks. What would the iron source have been, corrosion from my pump perhaps? I noticed when I first got the pool that someone had been putting individually-wrapped something directly into the skimmer- and not even bothering to do more than open the packet, as 5-6 torn plastic wrappers were collected in there. Pretty sure that they didn't blow into the pool from somewhere else since the pool had been kept covered.