stains on pool floor

[jtindisguise]

Greetings all,

I'm new to this board. While vacationing a few weeks ago the pH level of my pool went too high. To make matters worse my salt cell shut off due to the high PH and scale build up. I had the solar cover on when this happened the warm, dark environment made a nice green pool for my return. I got the salt cell going again and ran, ran, ran, ran the pool and backwashed out a few thousands gallons and the water returned to its clear self, however, the floor did not. I have thus far added a total of 3 gallons of liquid chlorine and been brushing daily and I cannot get the black streaks from the floor of the pool. They looks like sneaker scuff marks.

Here is the pool:
15,000gl
salt water
pebble sheen
in-floor cleaning

The water (tested today)
free avail. chlorine 4ppm
total avail. chlorine 4ppm
salt 3600ppm
cyanuric acid 30ppm
total alkalinity 110ppm
pH 7.5
phosphates 500ppb

I would like to not drain, acid wash, or hire a pool pro.

[muss08]

Have you water test for iron. There is a chance that when the pH ran high iron stains occured which range from dark brown to black. If you do have iron in the water you need to use a metal sequestering agent.

[jtindisguise]

I'm pretty sure its algae build up. After reading lots of post I dont think its iron. Also, I took the attachment of my telescoping pool and poked one of the black spots. It was a bump on the floor, literally. After I broke the bump apart the black in that spot disappeared. Think I am going to try a weighted stainless steel brush because my current brush is doing nothing. I also lowered the pH to less that 7 to aid in my brushing.

[muss08]

Okay, sounds like you have black algae which is the nastiest kind. It forms a protective shell (the bump) and it must be vigorously brushed while keeping your pool at shock level until it is gone. Mustard/black algae shock level for a pool with a CYA of 30 is 19ppm. To get from 4ppm to 19ppm it will take about 5 96oz. jugs of 6% bleach but you will need more to maintain that level until it is gone. You need a test kit that can test high chlorine levels such as the Taylor K 2006. Most others only test FC to 5ppm.

[jtindisguise]

It does seem like its fading and not so noticeable. Sounds like I need some more chlorine! I'll keep shocking and brushing. The brush I am using seems to just move water. Will this much chlorine effect my salt cell though? Isn't chlorine corrosive to metals? Also, once I ditch the algae do I need to purge some water to get back to normal or just the it evaporate off? I live in Arizona where the sun evaporates water pretty quickly.

[muss08]

Your salt cell will be fine, dont worry. Chlorine is corrosive at very high concentrations for extended periods of time. I would get your pH back up to at least 7.0 because the lower pH will be more corrosive. When the algae is cleared up, turn your generator off and the chlorine should burn off in a day or two. Not to worry. Just some advice- with a generator I would suggest raising your CYA to 60-80ppm so you dont have to run it so much and get your TA down to 60-80 because you will be constantly fighting a raising pH with a generator. A lower TA will slow this somewhat.

[jtindisguise]

Thanks for the input. Last night I put in another gallon of liquid Cl and used a stainless steel brush. The brush had a huge impact. Almost immediately the black algae stains were fading. While the cl content is really high I will continue to brush to make sure I dont miss anything and then get the water back in order. Thanks for the help.