Swimming Pool Blog

Why is my Pool Still Green?

Why is my Pool Still Green?
by Mark Garcia, June 29, 2017

We hear it all the time... "My chemistry is perfect, I've shocked the pool, added algaecide, running the filter, but it's still green! Help!?!"

Let's break it down, shall we? 

1. "My Chemistry is Perfect"

Perfect water chemistry would be pH 7.4-7.6, Alkalinity 90-110 ppm, Calcium Hardness 200-400 ppm, Cyanuric acid 20-30 ppm, Free Chlorine 2-4 ppm, Combined Chlorine <0.3 ppm, TDS <1500 ppm, Phosphates <300 ppb. If you haven't really tested all of these parameters, and made any adjustments needed, it may not be so perfect as you think. And, to test accurately, use a good test kit, like the Taylor K-2005 or K-2105. Low levels are as bad as high levels in most cases, but for fighting algae, you specifically want a lower pH and Alkalinity, and Cyanuric Acid (stabilizer) that is not too high. This helps chlorine to be most effective. At a pH of 8.0, nearly 80% of your chlorine can be inactive, sluggish and slow. High cyanuric acid levels also suppress chlorine activity. Phosphates can be tested with a $10 phosphate test kit, and if levels exceed a few hundred ppb, treat the pool with a phosphate remover, like PhosFree or Seaklear.

2. "I've Shocked the Pool"

Many people follow the dosage instructions on the bag of shock, usually 1 lb. per 10,000 gallons, and wonder why the pool is still green. 30 ppm of chlorine is often needed, using 4 lbs per 10,000 gallons to reach 30 ppm, to kill very active algae blooms. In some cases where the algae is very thick and visibility very low, it would not be uncommon to need 5 or 6 lbs per 10,000 gallons. If you have shocked the pool, and it is still green, you missed the mark. Just keep adding shock until the water turns a blue-grey color, with no hint of green, then add another pound just to be sure. This 'blue-ing' should occur within 5 or 10 minutes of adding the shock, if not, add more. Add your shock in the evening, when the sun is no longer shining on the pool, and be sure that the pH level is 7.2-ish, for best results. The following morning, the pool should still look blue-grey (with no hint of green), AND - you should still show a testable chlorine level in the water. If the chlorine level drops to zero within 24 hours of shocking, you missed the mark. Pool Shock is expensive, I get it - but if you really want to kill the algae, hit it hard!

3. "I Added Algaecide"

Algaecide is better for prevention, not so good at killing algae. Rather than being a true algae-cide, they are more of an algae-stat, used for control or suppression of algae. And, if you add algaecide and shock chlorine at the same time, the shock will destroy the algaecide, or render it inactive. To kill algae, use lots and lots of chlorine (see above), and then after the chlorine level has come into normal levels and the pool is beginning to clear (usually 1-3 days), you can add an initial dose of algaecide, and weekly maintenance doses thereafter.

4. "I'm Running the Filter"

Are you running the filter 24/7? That is what we would recommend when trying to clear up a green pool. It's often not even mentioned by customers seeking help, until later when we discover that the pump runs only 8 hours/day, and only at night :-(  A big part of algae prevention and algae cure is to keep the circulation (pump) and filtration (filter) working hard, so your pool chemicals don't have to shoulder such a heavy burden. Run your pump long enough each day to keep the water clear, and run during the day when the sun is beating down on the pool; when photosynthesis is happening. For properly sized pool filters, 10-12 hours per day is possible, but for undersized and underperforming filters, running it less will only cause you more water problems. Small filters may need 18-24 hours daily, especially during hot weather, as a prevention against algae. Be sure to use a pump timer, don't rely on your memory or availability to turn on/off the pump each day. That will fail you one day, and can be all it takes to start an algae bloom.

So, how's your filter doing? Is it running long enough each day, and is it trapping dirt, raising the filter pressure? Resist the urge to backwash too often, wait until the pressure gauge rises at least 5 psi, or flow is visibly diminished. And be sure that you are backwashing long enough, with sufficient flow and discharge, or cleaning the cartridge properly or thoroughly. For sand or DE filters, backwash valve parts or internal filter parts problems can cause ineffective filtering, or allow water to bypass the filter, so be sure your filter and valve (multiport or push-pull, not used on cartridge filters) is operating properly, and that small debris is not bypassing. Slime Bag can be placed on return lines for improved filtration, or a Pool Clarifier can be used a day after shocking the pool, to help a struggling filter capture very small particles. In many cases where pools are still green after spending hundreds of dollars on chemicals, the filter sand or filter cartridges needs replacement, or the DE grids are clogged or damaged. Filter Cleaners can rejuvenate filter media, but there comes a time when it needs to be replaced, just like an air filter. Maybe that time is now?

5. "Been there, Done that, Pool Still Green

OK, so if you have checked and double checked all of those things above, and the pool is still green, and not getting better, I can only suspect that the filter is not operating properly, or the water is so choked with solids that it is overwhelming the filter, or both. Assuming that chemistry is 'perfect', the pool is clean and not filled with debris, and phosphates and nitrates have not contaminated the pool...

1. Replace filter media (sand, cartridge, grids), or install a larger pool filter

2. Replace pool water, even just half of the water, can make a big difference



I hope that this post will help out those wondering "Why is my pool still green!?!", in most cases it is:

1. Not enough shock chlorine used to kill the algae

2. High pH or stabilizer levels, or other water chemistry problems affecting chlorine strength

3. Not running the pump and filter long enough each day

4. Phosphate or nitrate contamination in pool

5. Filter media is deteriorated, needs replacing

6. Filter is too small, overwhelmed by the amount of solids

7. Filter or backwash valve is damaged, allowing bypass

8. Pump is drawing in excessive air, reducing water volume

9. Water is choked with solids, very high TDS levels

10. Chlorine is very old, and has lost potency over time



Thanks for Reading!

Mark Garcia