Swimming Pool Algae ~ Identify & Destroy

by John Galcius, September 17, 2009

green pool

The key to algae control, is prevention. If algae doesn’t get started, it won’t be an issue. My number one rule is chlorine. Chlorine to prevent the algae from starting, and if it does get started, chlorine to kill it. In most cases, it is very easy to prevent algae. Keep the chlorine level at 1-3 ppm at all times, keep the pH between 7.2 and 7.6 and shock the pool every other week. An algaecide can be used in conjunction with any maintenance program.  Also, as part of any pool maintenance program, brush the pool on a regular basis. If you allow the chlorine to go down to 0.0, even for a day or so, then bacteria will begin to develop and algae can bloom very easily. So, to prevent algae from growing, keep a chlorine residual of 2.0 ppm, balance the pH and add a maintenance dose of algaecide on a bi-weekly basis. And brush the pool - weekly. It’s that easy.

If you are doing this already, and you still have algae - you may have a high level of phosphates or nitrates in the water. These can enter the water from a variety of sources, one being wind blown fertilizer. If you have pool side planter areas, make sure that during a heavy thunderstorm that they do not overflow into the pool. Fertilizer and other chemicals good for plants are also good for algae. Treat with a Phosphate remover like Orb-3.  

OK, So you let the chlorine go down, and now the pool has algae. What to do? Well, there are several different types of algae and each one has different characteristics and each requires a different remedy. Let’s take a look.

Green Algae

Green Algae is the most common of all pool algaes. It usually starts with a green tint to the water, or a small green patch on the wall. If not stopped, it can spread fast and get thick. If the conditions are right, pool algae blooms will multiply by the hour. This type of algae is very easy to stop and kill in the early stages. Chlorine is the key. Lots of chlorine. Lower the pH and shock the pool. Be sure to add enough chlorine to kill the algae. Double your normal shocking dose, if you normally add 3 pounds of shock, then you will need to add 5 or 6 pounds to get rid of the green. Make sure to follow all directions on the packaging, and dilute the shock before adding if necessary.

The biggest mistake people make when adding chlorine to the pool when trying to kill algae is simply not adding enough chlorine. If the pool is 30,000 gallons and you add 3 pounds of chlorine in an attempt to kill algae, then you just wasted all of that shock. The pool will still be green, and you’ll be out 3 pounds of chlorine. Always be sure to hit the pool hard enough with chlorine when trying to kill algae. If the shock is only on the wall, in isolated areas, pour the granules down the wall - in that area.

If the algae is blooming all over the pool, looking like pea soup - then broadcast your shock over the water, evenly distributing the granules. Remember to lower the pH first, as chlorine is much more active at a pH range of 7.2 - 7.4. Keep shocking the water until the pool turns a blue/gray color. (remove solar blankets and pool cleaners to protect them from the high chlorine). Then Brush! The other mistake people make is they don't brush thoroughly enough. It can be difficult, tiring - and you need a good brush. I personally think that all plastered pools should be brushed weekly. It's great for the pool, and great exercise for you too. And, it can help prevent green algae from returning, by keeping the small pores of your plaster clean, and allowing your algae fighting chemicals to make close contact in these areas.

Black Algae

Black algae are very distinctive and can be very difficult to remove. Small black dots, or marks on the pool floor and walls. Starting small, and getting as large as a dime, these spots will not brush off easily and look black, but are really very dark green. The best way to kill and remove black algae is to rub a chlorine stick or chlorine tablet over it. The heads of the black alga need to be scraped off, to allow chemicals to penetrate to the roots. A pumice stone or even a putty knife can be used initially, followed by a good brushing with a steel bristled pool brush. Brush the spots vigorously. It may seem like the brush isn’t doing anything, but it is. As the brush hits the algae blooms, it will open the spores, and allow the chlorine to attack.

Step two is similar to the first, lower your pH and shock the pool with a heavy dose of chlorine shock. Calcium Hypochlorite or DiChlor shock is good, and if you can find it, granular Trichlor is perhaps the best way to kill black algae. Whatever method you choose, several treatments might be necessary before all of the black algae is gone. Keep a close eye on it, and attack the areas that bloom first.

Step 3 for black algae treatment is to replace the filter media. New sand, new cartridges or new grids. The spores of Black algae can survive within your filter tank, and regenerate later. Step 4 is to keep a maintenance dosage of a good algaecide in the water. I like a combination of 7% copper algaecide, plus Poly 60.

For extreme cases of Black Algae, a drain and acid wash followed by a chlorine wash (never allow chlorine and acid to mix!) is indicated. Make sure to check behind the light and under ladder treads. This algae likes to hide. Even a drain and clean will many times not remove it completely and it can return if not suppressed effectively. In these cases, an extreme remedy would be to replaster the pool, essentially burying it alive.

Mustard Algae

Appears as orange, yellow or mustard spots on pool walls and floors. It will brush off very easy, almost like powder, but can return sometimes in a matter of hours. It often forms on the shaded side of the pool, or behind ladders and can be very difficult to remove. The best algaecide for mustard algae is usually copper based, I find, and when treating the pool for mustard algae, you need to treat the pool tools as well. Before adding the mustard algae treatment to the pool, put the tele pole, the skimmer net, the vacuum hose, the pool toys and anything else that has been in the pool during the mustard algae bloom. Even bathing suits. That will insure against a re-infestation of mustard algae. Also important is the filter media. Replacing the sand is a good idea. Grids or cartridges should also be replaced or cleaned very well with a filter cleaner, followed by a soak in a chlorine solution.

Most of the available yellow algae treatments are not actually algaecides, but chlorine boosters. Brands like Yellow Out or Mustard Buster will ask you to lower the pH, and shock the pool, followed immediately with the chlorine booster chemical. So you see, like other types of algae, chlorine is one of the best algaecides that can be used.

Pink Algae or Pink Slime

Often seen around ladders and pool lights, and in corners. Usually found where circulation is minimal. Baquacil pools are notorious for pink algae and you can see it on most water fountains in public places. Pink Algae brushes off very easy, but hard to kill. Pink slime is not really a form of algae. It is animal not vegetable. It can be pink or red in color and forms a heavy, protective slime coating that protects it from enemies. However, pink slime is treatable. The key is a combination of manually removing it, and then using a combination of polymer pool algaecide and chlorine. Add these in separate treatments, because shocking the pool will break apart the polymer chain in a Poly 60 or Poly 30 algaecide, rendering it useless. Lower pH, shock and brush, then treat with a kill dosage of Poly 60 2-3 days later. Make sure to get it off of the ladder, the light niche, the pool light and any other corner it can hide in. Pink algae can also be brought into the pool by swimsuits that have been used near the ocean. Good idea to switch suits or bleach these heavily.

Pools tend to have algae problems -or- they don't . If you have easily managable algae, consider yourself lucky. Using these steps above will help you keep your algae under control. It will always be there, we just keep it suppressed. If suppression becomes increasingly difficult, try a phosphate remover. Draining and refilling the pool every 5-10 years is not a bad idea in these cases, if feasible.

Death to Algae! Power to the Pool Owners!