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Help for Dark Green Algae Filled Pools

What some people don't realize is that an algae filled pool can damage your pool surfaces. Stains of course, but over time, algae and poor water chemistry will begin to erode the pool plaster, with etching and deposits, and weaken vinyl liners, making them brittle and old before their time.

If you just now took the cover off of the pool, in late June, and you have a dark green pool - there are some good solutions for you below. If this is more than just a late pool opening, but a pool neglected for months or years - we have solutions for you also.

Neglected or abandoned pools are also a problem from a health and safety standpoint. When not protected by a safety cover, they become a safety hazard for humans and animals, and stagnant water will attract mosquitos which could bring disease to the community.

To Drain or Not to Drain? 

Algae filled pools can have varying levels of suspended material, and debris on the bottom and floating on top. If you can see the shallow end floor, even if just barely - you can probably bring it back, with lots of chemicals, cleaning and filtering. If you can only see down 12-18", and there is a lot of debris in the pool, trying to bring it back without at least partially draining will be a long road.

Drain Your Pool

A lot of homeowners are nervous about draining the pool, having heard horror stories of pools that have popped out of the ground. And yes, that can happen, but not if you drain the pool properly.

  • Rent or borrow a submersible pump, and discharge hose to reach the street, storm drain or downhill slope
  • Check the weather before draining, heavy rain storms increase the risk of hydrostatic pool problems
  • On plastered pools, when draining completely, open a few hydrostatic relief valves in the pool floor and drain
  • Plastered pools can be pressure washed or acid washed, Vinyl pools should use water and a soft brush

use a sump pump to drain your algae filled pool

If your algae filled pool has a main drain, you can usually drain most of the water out, using the filter pump. Just close off the skimmer valve(s) before the water level gets to the bottom of the skimmer, and let it continue to pump out the waste line, using the main drain. It's common however, that at some point before being fully empty, the pump will lose prime. At best, it will stop with about a foot of water over the main drain.

Draining the pool can save a lot of time and money. With a small submersible pump, and enough discharge hose to pump downhill or to a storm drain, you can drain most pools in 12-24 hours. Larger, gas powered pumps can also be used, which can drain a pool in 1-3 hours. A faster draining is preferred, to allow you to hose off the walls while it drains, to prevent dried on dead algae from baking in the sun. Use enough discharge and direct the water far enough away so that it's not coming to rest under the pool.

For Vinyl Pools

Drain it down to the bottom step in the shallow end and refill the pool. If necessary, repeat the process until your pool is mostly blue, or just a light shade of green. If you drain it down too close to the shallow end floor, the liner may relax and you may have trouble resetting the liner correctly again, so only drain about half the pool. The same rule applies to fiberglass pools, although maybe a 1/3 rule would be more appropriate.

For Homes with Wells

Many homeowners with wells are concerned that they may 'burn up the well pump', or 'drain the well' if they use it to fill a swimming pool. Last week a homeowner told me of his plans to pump water out of a stagnant pond, rather than use his well! I'm telling you, in most cases there is no need for concern. It's no different than running the lawn sprinkler all day, and I've never seen a well run dry filling a pool, in all my years.

Filling from a Truck

Many areas have a local service for pool water delivery, 5000 gallons at a time. This can be very expensive at about $250 per load, but for those with very weak wells, or water with very high mineral content, it can be a good option.

Water Restrictions

For those areas that have water restrictions in place which prohibit draining of the pool, there are pool water recycling companies that will pump out your pool water into their mobile reverse-osmosis filters, and refill the pool with clean and balanced pool water.Otherwise, follow these steps below.

Don't Drain Your Pool

Cleaning up a dark green, algae filled pool without draining may not be optimal, but if you are unable to drain the pool, here are some tips on how to successfully restore blue pool water. Depending on factors like water temperature, the amount of solids in the pool, and the capabilities of your pool filter system, the process can take 3 days or 3 weeks.

Start up the Filter

Without a properly working filter, the water will never clear. During the first week, you may need to backwash daily, or several times per day, and also empty the pump and skimmer baskets. Run the filter 24 hours per day. If you are worried about the filter pressure rising too high during the night, or while you are away at work, set a multiport valve on recirculate during these times.

Clean the Pool

This is the first step, vacuuming to waste, skimming, and brushing the pool. Allow the pool to filter overnight, and repeat the process the following day. Try to remove all of the debris, algae and dirt that is in the pool - before you move on. It will make the process much easier if there are not leaves and sticks on the bottom of the pool, and floating on the surface. Keep at it daily during the process.

Balance the Chemistry

The next step should also be given ample time, don't rush through balancing, but test the water, adjust as needed, and allow the pool to filter for 8 hours before re-testing and re-balancing if necessary. Keep an eye on the filter pressure, and backwash when it rises 8-10 lbs on the gauge.

Shock & Floc

In The Swim Pool Shock

With your pH on the low side, around 7.3, shock the pool with enough bleach or granular chlorine to turn the pool a milky shade of blue. Depending on the amount of algae in the pool, and other organic debris, you may need 2 to 4 lbs of granular shock or 2-4 gallons of bleach - per 10,000 gallons of pool water. Test your chlorine level the following day, and if it's low, or if the pool has returned to a greenish color, shock the pool again. Continue to filter the water 24 hours per day, backwashing as needed.

A few days after shock treatment, if your pool clarity has not begun to return, you may wish to use a pool flocculent like Super Floc to settle suspended particles to the pool floor. It's best to be able to vacuum to waste, through a multiport valve - the settled hydroxides form a gelatinous layer on the floor, which clogs up filters fast when vacuuming.

Brush the Pool

With all of your might, brush your algae filled pool vigorously. Repeat the process again every 8-12 hours, to kick up the dust, and continue to remove dead algae from the walls and floor. Brushing daily during the process will also help to remove stains on the pool surfaces. Vacuum the pool carefully to remove dead algae and shock dust. Vacuum to waste if you see it passing through the filter, or clogging up the filter too quickly.

Clean the Filter

Your pool filter can harbor the cells of dead algae, or not-quite-dead algae, which can cause the algae to return. Use a filter cleaner product to help flush out trapped material, or consider replacing filter sand or filter cartridges, after you have cleaned up the water.

Stain Removal

Many stains will lighten over the next few weeks, with high chlorine levels and plenty of sunshine. To accelerate the process, or for tough pool stain removal, you may want to use some pool stain remover chemicals, such as A+ Stain Remover.

In The Swim makes every effort to provide accurate recommendations based upon current ANSI/APSP/ICC-5 2011 (R2022) standards, but codes and regulations change, and In The Swim assumes no liability for any omissions or errors in this article or the outcome of any project. You must always exercise reasonable caution, carefully read the label on all products, follow all product directions, follow any current codes and regulations that may apply, and consult with a licensed professional if in doubt about any procedures. In The Swim assumes no legal responsibility for your reliance or interpretation of the data contained herein, and makes no representations or warranties of any kind concerning the quality, safety, or suitability of the information, whether express or implied, including, without limitation, any implied warranties of merchantability or fitness for a particular purpose.