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Blooming Ugly: Dealing with Less Common Varieties of Algal Blooms that Can Infest Your Swimming Pool

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Every responsible swimming pool owner naturally does their best to prevent the infection of their pool with green algae—these slimy mats of disgusting green ooze are unsightly, unsanitary and can provoke some nasty illnesses and skin reactions in swimmers. However, the other, more obscure varieties of algae that can affect your pool are often overlooked, and the measures pool owners take to prevent green algal blooms are not always effective at preventing the spread of these lesser known algae types.

With this in mind, here are some of the more uncommon algae types that may start breeding in your pool, along with methods of preventing their appearance and growth.

Black algae

What is commonly known as black algae is actually blue-green algae, a group of very hardy algae species that are quite resistant to chlorine, even in relatively high concentrations. Black algae does not affect the clarity of your pool water significantly, but instead tends to collect in slimy patches on the bottom and sides of a pool. This is perhaps the reason it is less well known than green algae since black algae needs a rough surface to adhere to properly, it is very rarely seen in vinyl or tile lined pools, and is largely limited to concrete and plaster lined pools.

Dealing with black algae is a three-step process:

  1. Analyse the pH of your pool water to make sure it is balanced at around 7. This can be done professionally or with a home testing kit. When the pH of your pool water is too high, the effectiveness of chlorine, cleaning chemicals and algaecide is reduced dramatically, allowing the black algae to spread much more freely. Balance the pH using ordinary pool balancing chemicals.
  2. Scrub, and scrub hard. If black algal blooms remain undisturbed, the layers of slime that form over the mats will protect the algae from chemical treatments. Use a stiff-bristled brush, and make sure the pool filters are running properly to catch loosened algae and detritus.
  3. Since black algae is resistant to chlorine, you will need to increase the chlorine concentration in your pool dramatically—ideally between three and five times your normal chlorine concentration. This is often referred to as 'shocking', and will quickly kill the algae. Optionally, you can also add algaecidal treatments for added oomph.

Mustard algae

A form of algae related to green algae, mustard algae is a yellowy-brown tinge, and tends to collect in sparse deposits on the bottom of a pool that often resemble sand or dirt. For this reason mustard algae is frequently misdiagnosed as a problem with high mineral concentrations in the pool water, and pool owners can waste a lot of money on mineral filtering treatments before the realise the true cause of the problem. The easiest way of determining whether or not you have mustard algae is to feel the deposits: mineral deposits will feel gritty to the touch, while mustard algae will feel characteristically slimy.

Treating mustard algae effectively involves much the same process as treating black algae. However, you should be aware that mustard algae is far quicker at recolonising recently brushed areas if it is not killed outright, particularly in 'dead spots' in your pool where water flow is low or non-existent. Mustard algae also collects more readily in shaded parts of your pool, so you might consider removing shades or parasols that obscure your pool, or trimming back overhanging branches.