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Remember those old black-and-white monster movies? Some gargantuan beast attacks the city and the Army shows up to blast it. Everyone is celebrating the victory when all of a sudden, the menace rises back up from the ashes, bringing more monsters with it.

Dealing with algae in a swimming pool can be a lot like an old “B” monster flick. Operators use all their best chemical weapons, and the algae go away. However, a few weeks later, the invaders have returned. This begs the question: Were the algae completely eradicated or did some remain behind to proliferate?

Many aquatics professionals say that resistant algae are a fact of life. They swear that even though the affected pools are carrying residuals of chlorine and have been treated with algaecide, algae remain in the water. On the other hand, the opposing school of thought argues that as long as you maintain the chlorine residual and treat with an algaecide, you will not have algae.

Which side is correct? Let’s take a look at the facts.

First of all, you need to define resistant algae. Various types are found in pools, including green, yellow and black. Each has distinct characteristics. For example, yellow and black algae are able to secrete a substance that forms a durable sheath, which can protect them from toxins.

The American Society for Microbiology published a study in 1964 examining the relationship between algae in pools and the amount of algaecide needed to suppress them. The report concluded, “It is apparent that algae can develop which are resistant to the application of certain toxic chemicals.”

The study, “Factors in the Testing and Application of Algaecides” by Gene P. Fitzgerald, also stated, “Any toxic material that can be prevented from penetrating through the barriers formed by such algae colonies will be ineffective.”

In fact, many pool professionals have reported cases of algae with resistance up to 40 ppm.

What causes this resistant form of algae, and how can it be removed? When you’re dealing with any type of algae, it is important to understand what contributes to the organism’s existence.

The following are the most common factors affecting the growth of algae in pools:

  • Sunlight.

Algae are incredible minifactories that are able to use the energy from the sun to convert nutrients such as nitrates, phosphates and carbon into food through photosynthesis.

  • Lack of sanitizer.

It should be obvious to aqua-tics pros when a body of water doesn’t have enough sanitizer. Still, there can be special circumstances, such as when chlorine demand is unusually high due to hot weather, high bather load and heavy rains. When the sanitizer level is compromised, algae can get a foothold and flourish.

  • Warm water.

Most algae love warmer water. That’s why we see more of the organism in spring and summer as the waters warm up, and swimming activity increases.

  • Rough surfaces, cracks and crevices.

Rough plaster, cracked surfaces or tile lines make it easy for algae to hold on and grow. While they don’t have roots, species such as black algae have the ability to burrow into plaster.

  • Poor circulation.

Algae thrive in still waters. They can settle in areas where there’s little or no water flow. Improved circulation and a good filter system can keep algae from establishing residence.

  • Carbon dioxide.

Like other plants, algae breathe in carbon dioxide — and there’s plenty of it in pools. Some of the carbon dioxide is readily available from the atmosphere, and more is formed whenever acid is added to the water.

  • Nutrients.

These are the environmental factors that make the presence of resistant algae possible. Nutrients such as nitrates and phosphates contribute to algae’s ability to create food and flourish even in maintained pools.

Now that the “beast” has been identified and its growth factors revealed, we can work on getting it out of the pool. Use this three-step process:

1 Ensure equipment is in proper working order. Make sure the pump is working correctly and the filter is clean. If resistant algae are thriving in the pool, kill them by superchlorinating.

After the chlorine has been added, use a clarifier to help gather dead algae and brush down the entire pool. Run the filter system and backwash when the pressure rises.

2 Vacuum any dead algae from the pool floor. The next day, brush the entire pool again and backwash the filter if necessary. Add a good broad-spectrum algaecide (one designed to treat all types of algae) and set the filter to run six to eight hours.

3 Test the water for phosphates. Treat the water with a phosphate remover if the level is more than 200 ppb. Nutrient removal is an important part of an algae-fighting regimen. Maintain the phosphate level below 200 ppb.

This is especially important in pools with salt-chlorine generators. Phosphate levels above 500 ppb can cause bonding to the generator cell and reduce the output of free available chlorine.