Pictures to the left: Mustard/Yellow algae, black algae, green algae. 

Algae is introduced into your pool in various ways. The most common is simply from the wind, but it can come from swimsuits or maintence equipment (brushes, nets, pool sweeps) that have been exposed to algae. Algae loves still, warm water. That is why it seems to grow on the steps and on the wall that gets constant sun, it is warmer there.

1. Yes, the chlorine in your pool has to be maintained at 1-3 ppm (and all of the other chemicals balanced). All it takes is one day for the chlorine to drop below this level or for one of the other factors listed below to happen, and you can get a algae bloom. It is a good idea to have a constant source of chlorine in your pool: a floater with tabs, in-line chlorinator, or a chlorine generator (salt pool).

2. Filtration, filtration, filtration. If you have a filter cartridge (as most swimming pools do now), you need to be running your pool 10-12 hours a day during the day light in the summer (half that in the winter). Yes, I know you don't want to hear this because of the electric bill, but water has to be moving and filtered to prevent algae.

Every gallon of water in your pool should go through the filter at least once in 24 hours. Perhaps your filter is too small for this to happen even in 24 hours. Old recommendations used to be bigger pump to move more water through a smaller filter. Now, we know that most pools need only a .5hp to a .75hp pump, but a bigger filter - usually between 300-400 sqft to keep a pool clean effectively.

3. Run your pool sweep, or get a pool sweep and run it during the day as long the regular filter is running. It helps move water in "dead spots" that all pools have. Plus, it helps pick-up or stir-up that debris, so it can be filtered out.

4. Filtration part two:

a. Water has to be kept above the skimmer line or water can't filter -get an autofill (yes, they make ones that hook up to your garden hose and will keep the water always at the right level).

b. Lines need to be clear of debris and not clogged: common causes are pet hair, pine needles, pool toys.

c. Filter cartridges have to be clean (at least every six months) and in good condition (most manufacturers recommend replacing filter cartridges every 1-2 years; you may be able to get three years out of them).

d. Pump pot and skimmer baskets must be emptied when there is debris in them, otherwise water can't move through them.

e. Debris needs to be removed from the pool: netting, pool sweep, or vacuuming everytime there is debris in there.

5. Brushing every surface of your pool 2-3 times a week keeps algae from having debris to cling to or a place to grow.

6. Keep organic matter out of the pool: dog hair, fertilizer, leaves, grass and dirt. These things all contain phosphates (as well as source and rain water), which is algae food. Using a phosphate remover monthly as part of your maintence can help cut back the amount of algae blooms.

7. Use an algaecide weekly as a PREVENTATIVE measure. Only 4-6 ounces a week will help keep your pool clear even if you slack a little in the above areas.

 As you can see algae is not simply a chemical problem. Battling algae to get rid of it can take weeks, but it can be done. The best way to keep a sparkling clear pool is to take steps (like the ones above) to prevent algae from growing in the first place.