Pool is clear but bottom looks spotty and dirty

[BigDave]

It makes sense that the overnight FC wouldn't change much with so little of the algae's surface area left exposed to chlorine.
I think Borate is a good choice - maybe a little higher. The pH might be getting a tad high (especially with 400 CH). Muriatic Acid is a good way to bring it back down - Be sure to read and understand:Using Muriatic Acid to Safely Lower Your Pool's pH.

[rkrgk]

It makes sense that the overnight FC wouldn't change much with so little of the algae's surface area left exposed to chlorine.
I think Borate is a good choice - maybe a little higher.

Yeah, I was following PoolDocs writeup on increasing to 50ppm, I got only 5 boxes of borax last time, I am planning to add 5 more, thats the reason my pH was a little off too.

The pH might be getting a tad high (especially with 400 CH). Muriatic Acid is a good way to bring it back down - Be sure to read and understand:Using Muriatic Acid to Safely Lower Your Pool's pH.

I did reduce the pH to 7.4 in the last two days.

I also got the polyquat 60 from amazon. And planning to add and see if it makes any difference. and may be get some trichlor powder and sprinkle directly on top of the black spots and brush if I can.

[BigDave]

Sounds like a pretty goo idea - it will lower the pH locally so be on the lookout for pitting in the plaster.

One method I've read is to put the chemical in a zip-loc bag to transport it to the stain then pour / squeeze the slurry onto the target spots.

[rkrgk]

Sounds like a pretty goo idea - it will lower the pH locally so be on the lookout for pitting in the plaster.

Good point,yeah I somehow thought Trichlor is basic, when you mentioned I checked the Taylor booklet,its pH is 2.8-3.5. Hopefully wont stain, I guess I need to take a chance *rock and a hard place*.

One method I've read is to put the chemical in a zip-loc bag to transport it to the stain then pour / squeeze the slurry onto the target spots.

Sounds like a good idea. Let me go check if I can get some Trichlor powder locally here. I have the pucks from last year, but dont have a holder (like this, but got bad reviews.. http://www.amazon.com/Pentair-542068...pentair+holder) or else I could just use it to scratch the floor with it to get same effect I guess. shallow ends I could srub with the puck and see if it makes any difference. I will give it a try and let you know if it makes a difference.

Also do you know why it is bad to use ammonium hydroxide with chlorine in the pool? except that pool is unusable for few weeks. I read somewhere its effective against black algae.

[BigDave]

You could break up a puck and put the chunks over the spots.

I wouldn't advise ammonia products in a pool - PoolDoc or ChemGeek may be able to advise you better - I just don't have the depth or chemistry. I do know that you'd be likely to form chloramines and that monochloramine is an effective algaecide - is does stink and irritate skin. If you want to use the pool, I'd stick with shock FC levels and wear old swimsuits.

[chem geek]

The main reason ammonia products are used in a pool is that they form monochloramine that doesn't bind to CYA so in a pool with high CYA levels it's a way of "cheating" to get a higher active oxidizer level. This is especially true against algae that tend to look at monochloramine as if it were ammonia which is something they can use as a source of nitrogen, but the monochloramine kills them. However, your pool doesn't have very high CYA so shocking with chlorine should be quite effective. The key for black algae is to scrape off the waxy surface layer (the "heads") so that the chlorine can kill the algae below. Some people just maintain their pools at a somewhat higher FC level and brush regularly and the algae will slowly get killed while others go to shock level to try and get it over with more quickly -- up to you. At Ben's recommended FC/CYA levels, neither green nor black algae will be able to grow (yellow/mustard algae requires higher levels so is usually best to get rid of completely, if possible).

[rkrgk]

Thanks Chem Geek. I kept at shock level (15ppm+) for more than three weeks with daily brushing (yes daily, sometime twice a day) and for the last two weeks (of those three weeks) kept it at +shock+ (30ppm) level, still I could not get rid of all (or visibly none I should say) the black algae. Now I went back to keeping about 5-10ppm FC, since it got to a point where I had to say " I surrender" . So I was looking at any other options I may have, to remove them before I resort to acid wash (if thats the only way). I feel that I have a higher chlorine demand because of the black alage on the walls, they seems to slowly grow as time pass (unless I keep shock level chlorine levels as per Bens chart which is not practical for more than a month or so). I think I wasted a lot of money trying to figure this out, but no real end in sight **tired** So I am a bit desperate to know what I am doing wrong with my approach. I am sure you guys, chem geek/PoolDoc have seen and encountered all kinds of these issues in and out, either I am not giving enough info or there is no possible recourse with my issue(black algae) using BBB or I don't quite understand it. Thanks BigDave too for trying to get to the bottom of my issue.