Pool still green

Algae problems in swimming pool water.
Green (cloudy) water or slimy pool walls.
Black algae. Mustard algae. Pink or white pool mold.
chem geek
Pool Industry Leader
Pool Industry Leader
Posts: 2381
Joined: Thu 21 Jun, 2007 21:27
Location: San Rafael, California

Postby chem geek » Wed 11 Jul, 2007 22:02

For all of these posts, none of you tested or posted your pool's Cyanuric Acid (CYA) level. The level of chlorine you need to kill green algae depends on the CYA level. You cannot always trust pool stores with their numbers and you cannot trust cheap test kits including test strips (with a few exceptions for testing salt and borates). Get a Taylor K-2006 test kit from Taylor here or from Leslie's here or the even better kit based on the Taylor kit from tftestkits here.

If your CYA level is high (well above 50), then it will be very hard to kill the algae fast enough with chlorine alone and will take a whole lot of chlorine to do so -- in that case, doing a partial drain/refill while still adding at least some chlorine to keep the algae at bay is appropriate. At more reasonable CYA levels, then adding enough chlorine to get the Free Chlorine (FC) level to 40% of the CYA level is appropriate and you need to test and add more chlorine to maintain this level several times a day (as much as possible) so you stay ahead of the algae growth. See this link for how a pool can be cleared of algae using chlorine alone (this example is upon opening in the spring).

You will need to also physically remove the algae if there is a lot of it and that is done through frequent backwashing or cleaning of your pool filter assuming you have good circulation in your pool. If the algae settles to the bottom after it is killed by the chlorine, then you can vacuum to waste. You may also need to use OMNI Liquid Floc Plus if your pool has poor circulation or doesn't have a floor drain and would then vacuum to waste after using the floc.

You need to maintain the shock level of FC until the following three things all happen: 1) the pool is clear of algae (crystal-clear, not just dull or cloudy) and 2) you measure a minimal drop in the FC level overnight (not during the sunlight hours) of less than 0.5 ppm and 3) you measure minimal Combined Chlorine (CC) of less than 0.5 ppm.

To maintain an algae free pool, you should target an FC level that is 11.5% of your CYA level (an absolute minimum FC level of 7.5% of the CYA level). Realize that Trichlor pucks/tabs and Dichlor powder contain CYA in them. For every 1 ppm FC added by Trichlor, you also add 0.6 ppm CYA. For every 1 ppm FC added by Dichlor, you also add 0.9 ppm CYA. Also, for every 1 ppm FC added by Cal-Hypo, you also add 0.7 ppm Calcium Hardness (CH) though in percentage terms this isn't as rapid an increase (since CH levels are much higher than CYA levels). Unscented bleach or chlorinating liquid do not add extra CYA or CH.

Unfortunately, the chlorine sources without CYA or CH do not dissolve slowly so you need to add the chlorine every day if your pool is in direct sunlight. If you have a pool cover that is opaque to the sun's UV rays, then you can usually add your chlorine 2-3 times per week. The Liquidator is a product that some pool users have started using and report success for automatically dosing bleach or chlorinating liquid into their pool.

When adding most forms of chlorine or acid to a pool, do not add it quickly. Add it very slowly over a return away from the side (holding arm outstretched) to allow for thorough mixing. This is especially important in a vinyl pool with no floor drain. In some cases, you can pre-dilute in a bucket of pool water and then pour slowly (for Cal-Hypo, for example, since it dissolves slowly).


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