Had a customer who continues to have problems with a green pool right after opening and obviously isn't too pleased. This was the case last year as well and he asked why is this case again this year?
In short, the answer is "high stabilizer" or cyanuric acid which in his pool was over 200ppm. That's excessive. Very excessive.
Cyanuric acid helps to reduce the chlorine loss by protecting the free chlorine in the pool from the sun’s ultraviolet rays which will slowly dissipate it over time. Areas exposed to high levels of sunlight should maintain 60 – 80 ppm. When you have levels of cyanuric acid above that, it binds the chlorine making it less effective as a disinfectant and thus slower acting to kill bacteria/micro-organisms or prevent algae. It will also increase cloudiness in the pool water and make your water more acidic. Because of all of this, commercial pools are closed by the city when the level goes above 80 ppm.
The difficulties he is having with algae are indicative of high stabilizer because quite simply his chlorine is locked up to such an extent that it is practically ineffective. Industry rule of thumb is that you need a chlorine level of 7.5%-15% of your stabilizer level for the chlorine to be effective. For a stabilizer of 200 ppm, this would mean keeping the chlorine at 14-28 ppm which is extremely high. Clearly we don't recommend this. But, if he runs his chlorine at a typical 3-5 ppm, he will be falling behind as it just won't be enough. And as the algae blooms, it will clog up his DE filter very quickly, causing even more problems. To make matters worse, the local big box store recommended using a copper-based algaecides as the quick fix but after time that too will cause problems as the copper builds up in the water to such a level that it reacts with the chlorine and ironically turns the pool green.
At the root of his challenge are chlorine tabs. The customer used them all last season and apparently at an excessive rate. Each tab he put in contained stabilizer and over the season his level grew and got out of hand. And in shocking his pool, it is unclear whether he used a shock that did not contain stabilizer. Given the levels he has, I would expect he used trichlor or dichlor based shock as well only making the problem worse.
So what should he do now.
First, At this point, it would be recommended that a portion of the pool water be replaced with fresh water, because the only way to lower the level is through dilution. But he can also take some other steps to reduce the chances that this happens again. Such as:
- Vac to waste whenever vacuuming the pool Simply put a running hose in at the same time to replace the waster water with fresh water.
- Use pool bleach (12% Sodium Hypochlorite) which is a "pure" chlorine (no stabilizer) and stronger than regular supermarket bleach. It's easy to use, won't cloud up the pool and will help to get your stabilizer down over time simply because you aren't adding anymore with tabs. Simply pour the bleach in and mix it like you would your coffee. If push comes to shove, you can use regular supermarket bleach (no perfumes or scents, just plain bleach) but make sure it is at least 8.5% Sodium Hypochlorite (dollar store bleach is much weaker).
- Make sure the shock does not contain stabilizer. Look for “Cal-Hypo” and stay away from any shock that contains “Trichlor” or “Dichlor”. Yes, it’s usually cheaper but it leads to this problem.
- Reduce the number of tabs. Tabs are not meant to simply replaced en masse on a weekly basis. You need to find the right level for your pool that maintains the proper levels of chlorine without spiking your stabilizer.