When treating a swim pool for algae, it is important that all parts of the pool receive a dose of algaecide. Some of the many types of algae can exist outside the pool and will re-infest the water when brought back in contact with the water. An excellent example of this is yellow, commonly called mustard, algae. These spores can survive outside the water for extended periods of time. It is important that when you treat for algae you submerge your cleaning equipment in the pool overnight, so that the algae on them is killed also. If you don't take this precaution, the next time you vacuum or skim for leaves you may re-infest your pool with algae spores.
You must also use the recommended dosage of algaecide called for by the manufacturer or risk not killing all the algae even though you do not see any. It is recommended that you follow up with a maintenance dosage of algaecide to keep the algae from reappearing. It is important to note that the algaecide used to kill existing algae may not be the same algaecide you use for preventative maintenance. Following the manufacturer's guidelines is required for the chemicals to do their job.
Another major cause of reoccurring algae, especially if it keeps appearing in the same places, is poor circulation. Often we find the return(s) of the pool (where water reenters the pool from the filter system) directed towards the surface of the pool. This is done to help the skimmers collect debris or just to give the pool water a moving effect. Unfortunately, this can have the effect of creating dead spots. Dead spots are areas where little or no water is circulating. Even with main drains, the returns pointed upward to the surface means little or no circulation on the bottom or lower areas of the walls. This results in little or no algaecide reaching these spots and algae never really being eradicated.
By redirecting your return(s) downward or sideways you can help eliminate this cause of reoccurring algae blooms. There is no set way of doing this; you just need to adjust the return(s) until you find what works best for you. You may also need to run your filter system more to increase circulation. Note: When treating for existing algae, run your system for 24 hours a day until completely gone.
Another way to increase the area of circulation is to run your automatic cleaner. Even when the pool is not dirty, it helps to bring clean, chemically treated water to all the nooks and crannies of your pool. Just running your cleaner once a week can make a big difference in preventing algae from reoccurring.
The best method for circulating water to those dead areas of the pool is inviting everybody over for a swim. Swimmers, especially kids, do a great job of moving water around your pool. And after all, isn't this what you got the pool for anyway?