by Rob Cox,September 22, 2009
Put your mesh pool covered pool to bed improperly, and you may find a mess come spring! Algae is common beneath a mesh cover, especially when the pool is opened in late May/early June. This article focuses on steps you can take to have a trouble free, green free pool opening next spring.
Mesh material covers do allow a small amount of sunlight to reach water - this is true. In suburban areas, pollutants, micro-contaminants, and algae spores and everything else in the air will wash into the pool during rain / snow.
In my opinion, this is the real reason that algae can grow in a pool. Sunlight helps, but the main cause is all of the microscopic junk that washes through a mesh cover.
Fear Not! This can be prevented. Simply use enough winter chemicals to keep any algae from blooming. Enough Algaecide, or Chlorine, or Enzyme. Or a mix of all three, especially during early fall and late spring. I like to use an enzyme and phosphate remover in addition to a good Poly 60 algaecide to keep most of the algae food neutralized. Your pool water should already be fully balanced before closing, and shocked well with granular chlorine several days before your closing date. Pool shock can break apart some algaecides, rendering them useless, so never shock your pool and add algaecide at the same time.
The time of year to make sure that your chemistry is working is March-June. Your closing chemicals will keep the water fresh through autumn. From December-February, most pool water in the snowbelt is too cold to allow algae to grow. But when the water gets over 60 degrees or so, algae will bloom in the absence of proper chemical levels. When the spring is warm, lift up a corner of the cover, and check for algae.
If you notice any algae, unbuckle the cover along one side of the pool, and flip the cover over on top of itself. Brush the walls to get some circulation and redose the pool with algaecide or enzymes or shock chlorine. If you can locate the floater, toss in a few more chlorine tablets. Check the pH while you are at it. Algae likes a high pH, so adjust if necessary to a range of 7.2-7.4.
Keeping the water level from rising too high will also keep water temperature lower in the pool during spring. If you notice a wet spot in any areas, the water may be touching the pool cover, this warms the water and prevents debris from drying and blowing off of the cover. This also creates a "tea bag" effect, allowing leaf tannins to leach into the pool water, which can stain some surfaces. Use a small cover pump or a siphon to lower the water down below the tile line. Water level under a mesh cover should not be lower than 18" from the cover.
Mesh covers are meant to fit tightly around the pool, to keep leaves (and kids) out. If you have flagstone or other flat coping around the edge, leaves may blow under the cover. Using water bags in certain areas, laid over top of the cover's edge, will prevent this. If your cover goes up and over a spa, or has to climb steps, this is another area to gusset up. Using pool noodles or other foam, or pieces of heavy material or inflatable pool toys, can seal up these gaps. Keeping leaves and critters out of the pool will keep your chemicals working to prevent algae growth, instead of attacking this other organic matter.
I have known some mesh pool cover owners that leave the cover on, but de-winterize the pool in early April. They run the filter just a few hours per day until they are ready to open it up in late May, after all the pollen and other spring debris flies. This also helps to spot problems with the equipment early, giving more time to make any repairs. Problems like leaking equipment or heater problems. But the main justification for this is that these pool owners can even open in June, without algae problems.
Mesh covers are great - beautiful, safe, low maintenance, durable. But they do have the drawback of allowing algae growth - if you let it! Completely preventable - just simply add another $20 worth of chemical, at the right time, and you'll save time and money later.