Algae growth is stopped at temperatures below 40° F, but some algae can continue to survive, and like weeds in a lawn, can go dormant over the winter, coming back to life in early spring, weeks before you open the pool.
1. Under what conditions do algae grow best? Algae grow best when they receive 10 -15 hours of sunlight a day and the temperature stays between 60-80° F.
Much like the veggies in your garden, when the water is cold algae won't grow. Algae is not happy if the water is below 50 degrees. The winter kit chemicals dissipate after a few weeks, but they help keep your chemicals balanced and the pool clear until your water turns into a popsicle.
If conditions are right, these spores can spread rapidly and take over the pool in a short amount of time. Warm weather, sunlight, lack of circulation in the water, and nitrates or carbon dioxide in the water can all be contributing factors for algae growth.
Pool algae can be caused by poor filtration, out-of-balance water, low or inconsistent chlorine levels, or poor water circulation. Green, dark green, yellowish green and blue-green algae are the most common. Green algae are slimy.
But is it safe to swim in a pool with algae? Whether mild or severe, it isn't recommended. Significant amounts of swimming pool algae welcome a breeding ground of harmful bacteria that feed on algae. These bacteria pose health risks to swimmers, most commonly resulting in a skin rash.
It is best to close your pool when the temperature of the water is consistently below 60 degrees. Once the water temperature drops below 60 degrees, microorganisms and algae cannot grow and become dormant for the winter season.
You can get rid of algae quickly by vacuuming and brushing your pool, balancing your pool's water chemistry, and then shocking and filtering your pool water. Just be thorough as you clean your pool surfaces. If you leave behind even a small number of algae spores, it won't be long before they regrow and bloom again.
Green Algae
First signs of it appear in small clusters on pool steps or lurking in corners. It's at this stage that you should start to attack it — green algae can grow quickly in 24 hours or less.
Closing a Pool with Green Water
No, not really. Why? Because closing a pool when its green is going to give you an even bigger problem in the spring or the next time you open it. When you are experiencing cooler, fall like temperatures, it takes longer for the chlorine and other chemicals to evaporate.
Keep it covered.
Covering the pool is a good way to prevent algae growth. The right cover keeps algae from entering the pool, but it also keeps leaves, bugs, bacteria, and dirt from decomposing in the pool, providing algae with food.
Warmer temperatures prevent water from mixing, allowing algae to grow thicker and faster. Warmer water is easier for small organisms to move through and allows algae to float to the surface faster. Algal blooms absorb sunlight, making water even warmer and promoting more blooms.
So while a solar cover won't actually 'turn your pool green', it will warm your water by up to 8 degrees, so if the other conditions are right, adding a solar cover can easily accelerate algae growth, very rapidly. You need to get the water balance in your pool right before putting the cover back on.
Pool water turns green because of algae in the water. Algae can grow rapidly, particularly when it's warm like Summer, which is why it can surprise you overnight. This generally comes down to an imbalance or lack of chlorine in the water.
A nylon or rubber brush is the correct choice for scrubbing the sides of a soft-sided above-ground pool. A large pool brush makes quick work of the job, but you may need a smaller brush to clean corners. Once the particles have been removed from the sides of the pool, turn your filter back on and agitate the water.
Grab a brush and some baking soda. Bicarbonate, the active ingredient in baking soda, is an effective spot treatment to help kill the algae and loosen it from the wall. Make sure you really get every last particle free; black algae has particularly long and stubborn roots which makes it a persistent strand.
Freezing temperatures
September and October are a good time to close the pool because the weather is ideal. It's still warm to be outside, enjoying the sun, even if you hate to say goodbye to summer. But once those temperatures start dropping significantly, you're going to kick yourself for not closing the pool sooner.
The month of March is the BEST time to open your swimming pool. If you wait any longer for the temperature to rise higher than 65 degrees algae and other organics will start to grow.
Definitely close your pool before the temperatures fall under 30°F (-1°C) at night. As long as the pump is running, the water is moving, so it won't freeze. But if you lose electricity during a storm and it gets down below freezing, the water will freeze in the pump and filter.
The pH may be too high.
If the pH becomes too high (over 7.8), it prevents the chlorine from doing its job. If you're not checking the pH and it has risen too much, you could be adding the right amount of chlorine, but it can't work correctly, and algae will begin to grow.
Proper chemical balance and sanitizer levels will prevent many opportunities for algae to bloom. High pH and low chlorine (or other sanitizer) can give algae a great start.
Baking Soda and Green, Blue, or Yellow Algae
You'll need to use an algaecide to kill the algae and superchlorinate your pool to clear the water. After this treatment, test your pH and alkalinity and add baking soda to raise alkalinity to at least 100 ppm and pH to between 7.2 and 7.8.
A fiberglass pool in its worst condition can be algae-free in 24 hours. For a vinyl liner pool, the process can take 3-4 days. For a concrete pool, this can take a week or more.