I think it is algae now....

[smallpooldad]

It might be better to bring your pH down to 7.4 as chlorine tends to work better at a lower ph, some say 7.2 but I am not sure if that is ok for a vinyl pool, so I indicated a pH of 7.4. To go from 7.8 to 7.4 you would need to add an additional 17.5 oz of Muriatic acid and also add 15 oz of baking soda to hold your alkalinity at 120, over and above the figures listed below, per every 10,000 gallons.

If you are adding liquid chlorine via 5.25% bleach to get to 15 ppm, you might wish to add approximately 7.53% Muriatic Acid, by volume of the amount of liquid chlorine. This is done to hold the pH at the starting point, assuming you are using 5.25% liquid un-scented bleach; otherwise the pH will rise and the chlorine will be less effective.

If using 6% liquid bleach you will need to add 8.73%, 8% Ultra Bleach (liquid) would need 11.78%.

So if you add 100 oz of 5.25% liquid chlorine you would add 7.5 oz acid, do this every time you add chlorine. Once the chlorine goes over 10 ppm it is harder or impossible to get an accurate pH reading. This would negate this problem. Non-liquid bleach numbers are different.

If you know how to use an Excel spreadsheet, and are technically inclined, the PoolSolutions spreadsheet is excellent for calculating your needs. It can be found here:

http://richardfalk.home.comcast.net/...lEquations.zip

As far as the future goes, once this algae is removed and based on a CyA of 50, you may wish to lower your pH to 7.5 (same as the eye) and and keep your FC at 6 (would kill most algae), you may get away with as low as FC 1.5. Holding a FC of 8 would nearly guarantee no return of mustard algae, see which works for you. At pH of 7.8 you need a little more chlorine (no biggy), minimum FC 2, FC 9 on the max side should assure no return of mustard algae. To kill most algae you would need FC 7.

Please note the above numbers are technically calculated numbers, and could vary from pool to pool eg. one pool may need, FC 7 to kill most algae, another only 5, and another 8. It all depends on your situation, wind, dust load, bugs, debris, leaves, etc.

The FC level needed may even vary at different times of the year depending on your location, the amount of shade your pool receives, and on your mean UV index, see here:

http://www.cpc.ncep.noaa.gov/product..._meanmax.shtml

It seems based on your problem that FC 2, is not a place to be so I would not go that far down at a pH of 7.8, or to 1.5 at 7.5 pH. At least not in July or August as the UV indexes are at their height as are temperatures, normally.

You maybe lucky that using the minimum number controls your problem, if it were me I would start at a FC 6 or 7, depending on where you want your pH to be, and lower the FC to the point where algae is not an issue, possibly no lower than FC 3, which is what the "Best Guess CyA Table" shows. If you raised your CyA you might need to raise the chlorine level, if you lowered the CyA to say 35 you might be able to lower them a little. Please note CyA can only be lowered by draining off water, something you may not wish to do. You will just have to try by trial and error which works best for you. Again it is best to refer to the "Best Guess CyA Table", here:

http://www.poolforum.com/pf2/showthread.php?t=365

An easy way to calculate how much chemical to use is here:


http://www.poolcalculator.com/


Hope this helps.

Aloha