Chemistry Balance

• A swimming pool's water is a balance of various chemicals and substances within the water. If certain chemicals or substances are out of balance, foamy water is often the result. For example, if a swimming pool's water is too soft, that means it's suffering from low calcium hardness. Pool water with low calcium hardness is susceptible to foaming. In order to eliminate foamy pool water caused by too-soft water, you'll need to raise calcium hardness by adding calcium chloride.
  
  

Lotions

• Swimmers who use tanning lotions and oils will often leave a residue of those products behind when they exit the pool. Once lotion and oil residue is left in a pool's water, it can froth up, when agitated, by splashing. The solution mainly lies in having swimmers use waterproof products. Pool water defoaming chemicals are also available, though they're normally used more so in spas.
  
  

Algae Decontamination

• Another culprit, when it comes to foamy swimming pool water, is an incorrect or inadequately applied algaecide treatment. Low-quality copper- or silver-based algaecides are susceptible to foaming up in a swimming pool quite easily. And even the best copper- or silver-based algaecides will foam up if too much is applied. Regular algaecide treatment of swimming pools, as a preventative measure, also isn't recommended because such chemical-treated water tends to foam up.
  
  

Multiple Causes

• Foamy water in a swimming pool is also often caused by more than just one single issue. For example, inadequate algae treatment, combined with air getting into a water return line, can easily cause foamy pool water. It's also the case that when foamy pool water is related to algaecides, the pool's filter will be clogged. Clogged swimming pool filters won't allow algaecide residuals and killed algae to be fully filtered out, which will lead to foamy water as well.
  
  

Considerations

• To help prevent foamy pool water, calcium hardness levels should be between 200 and 400 parts per million (ppm). Use 2.5 pounds of calcium chloride per 20,000 gallons of pool water to raise calcium hardness in 10 ppm increments. Non-copper- and non-silver-based pool algaecides are also available, including quat and polyquat algaecides, which don't cause foamy pool water. Lastly, always check all pool feeder lines to ensure no air is getting into them and causing foamy pool water.