What naturally kills algae in a pool?

Algae growth is a common problem for pool owners. While chemicals like chlorine and bromine can help control algae, some pool owners prefer to use more natural methods. The good news is there are several natural ways to kill and prevent algae without using harsh chemicals.

What Causes Algae in Pools?

Algae spores are always present in the air and water. When conditions are right, the spores multiply rapidly, causing an algae bloom. There are a few key factors that contribute to algae growth:

  • Sunlight – Algae needs sunlight to photosynthesize and grow.
  • Stagnant Water – When pool water is not properly circulated, it provides an environment for algae to thrive.
  • Nutrients – Phosphates, nitrates and other nutrients in the water feed algae growth.
  • Warm Temperatures – Algae tends to grow faster in warmer water temperatures.
  • Low Sanitizer Levels – Chlorine and bromine help kill and prevent algae. When levels drop, algae can take over.

By understanding what enables algae growth, we can take steps to control it naturally.

Natural Ways to Kill and Prevent Pool Algae

Here are some of the most effective natural remedies for combatting algae in pools:

1. Circulation and Filtration

Proper circulation and filtration is key to preventing algae in the first place. Pool circulation systems use pumps to continuously move water through the filter, removing particulates and debris. Running your pool’s circulation system adequately (usually 8-12 hours daily) deprives algae of the stagnant conditions it needs to thrive in.

Make sure your filter is properly sized for your pool volume and cleaned regularly. Built-up debris reduces flow and filtering efficiency. Consider upgrading to a variable speed pump if you don’t already have one – they can run longer at lower speeds to keep water moving and help prevent stagnation spots.

2. UV and Ozone Treatment

UV light and ozone generators are two supplemental pool sanitization systems that can help kill algae naturally:

  • UV Light – UV systems use special bulbs to emit UV wavelengths that alter the DNA of algae, preventing it from reproducing. This provides a means of controlling algae without chemicals.
  • Ozone Generators – Ozone is a naturally occurring molecule made of 3 oxygen atoms. Pool ozone generators produce ozone gas which is a powerful sanitizer and algaecide.

While UV and ozone systems require an initial equipment investment, they can provide effective and eco-friendly algae prevention in the long-run with minimal recurring costs.

3. Algae-Eating Fish

Certain types of fish are natural consumers of algae. Introducing a few algae-eating fish to your pool can provide natural algae control. Some good options include:

  • Chinese Algae Eaters
  • Siamese Algae Eaters
  • Plecos
  • Tilapia

The fish will continuously graze on algae, cleaning the pool naturally. Be sure to get fish appropriate for the pool size and environment. Provide adequate food so they don’t starve once the algae is under control.

4. Barley Straw

Barley straw has been used for centuries to clarify pond water and inhibit algal growth. When barley straw decomposes in water, it releases chemicals that prevent algal spores from attaching and forming on surfaces. To use barley straw:

  • Get untreated barley straw – avoid dyed decorative straw.
  • Place loose straw in mesh bags or an old sock.
  • Use about 6-10 straw-filled bags per 10,000 gallons of pool water.
  • Distribute the bags around the edge of the pool.
  • Replace the barley straw every 4-6 weeks through the swim season.

The decomposing straw will also help clarify the water. Barley straw is inexpensive, natural and safe for swimmers.

5. Hydrogen Peroxide

As an oxidizing agent, hydrogen peroxide (H2O2) can help kill existing algae. However, hydrogen peroxide also quickly decomposes into oxygen and water, so it won’t provide any residual sanitizing or algae prevention. Follow these steps to use hydrogen peroxide:

  1. Test and adjust pH between 7.2 – 7.6.
  2. Shock treat the pool to 50-100 ppm chlorine.
  3. Brush walls and floors thoroughly – this loosens algae for removal.
  4. Turn off circulation system and chlorine feed overnight.
  5. In the morning, empty skimmer baskets and clean filter.
  6. Add 1 pint of 35% liquid hydrogen peroxide per 500 gallons of water.
  7. Turn circulation back on for at least 8 hours.
  8. Resume normal chlorine regimen.

Repeat as needed for severe algae until growth is controlled. Hydrogen peroxide works best when combined with physical scrubbing and good filtration.

6. Natural Swimming Pool (NSP) Design

For a long-term, eco-friendly solution, consider converting your traditional chlorinated pool to a Natural Swimming Pool (NSP). NSP’s rely on the plants, rocks and gravel in regeneration zones to filter water naturally and prevent algae from taking over. This creates a healthy, self-cleaning ecosystem similar to a pond.

While the initial installation costs are higher, a properly designed NSP provides chemical-free swimming once established.

When to Seek Professional Help

For minor algae growth, the above natural remedies can often get the problem under control. However, if you have a major recurring algae issue, it may be time to call in a pool professional for assistance.

Signs it may be time for professional help include:

  • Thick algae biofilms on all surfaces
  • Green, black or mustard yellow algae
  • Cloudy water and inability to maintain proper chlorine levels
  • Failed attempts to eliminate algae using natural remedies

A pool service professional has specialized chemicals, tools and expertise to fully clean and treat major algae blooms. They can also determine if there is an underlying issue enabling the algae growth, such as:

  • Improper water chemistry
  • Insufficient filtration run times
  • Cracks, leaks or openings where algae can enter
  • Machine malfunctions
  • Structural deficiencies

Addressing underlying problems is key for long-term prevention. While calling a pro may have some costs upfront, it can save money compared to continually buying algaecides and dealing with equipment damage from improper water chemistry.

Preventing Algae Growth

An ounce of prevention is worth a pound of cure when it comes to pool algae. Here are some good habits to help avoid algae taking hold in the first place:

  • Maintain proper chlorine or bromine levels.
  • Regularly test and adjust pH and alkalinity.
  • Shock treat the pool weekly or as needed.
  • Run the filtration system sufficiently (8-12 hours daily).
  • Brush walls and vacuum regularly.
  • Clean the filter monthly or per manufacturer instructions.
  • Use a pool cover when not in use.
  • Address cracks, leaks or openings preventing contamination.
  • Limit nutrients by preventing runoff from fertilized lawns.
  • Keep phosphates from pool chemicals in check.

Vigilance and proper maintenance go a long way in keeping ahead of algae. Pay attention to potential blooms and treat them early before they get out of control.

Common Pool Algae Types

Knowing the most common algae types in pools and their characteristics can help identify and treat them promptly:

Green Algae

  • Most common pool algae.
  • Forms a green film on surfaces.
  • Usually occurs when chlorine levels drop.
  • Easiest to treat early before establishing mature growths.

Mustard Algae

  • Also called Yellow Algae.
  • Forms yellow-brown or mustard colored blooms.
  • More difficult to control than green algae.
  • Lower pH levels can inhibit growth.

Black Algae

  • Appears as dark spots or smears on surfaces.
  • Most difficult type to remove and control.
  • Can indicate old, neglected algae growths.
  • May require draining and scrubbing the pool surfaces.

Catching algae early before it becomes black algae will make removal and prevention much easier.


Algae can be frustrating, but there are many natural ways to get it under control. Maintaining proper water chemistry, circulation, filtration and sanitization is key. Supplement with UV, ozone, barley straw or other remedies for additional algae prevention. Persistent growth may require professional intervention to address underlying issues.

Vigilance and early treatment when first noticing algae will keep your pool sparkling all season long. With the right maintenance regimen, you can enjoy clear water without relying solely on harsh chemicals.

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