Algae does not form in the presence of adequate levels of free chlorine. Algae is an indication of poor chlorine level maintenance. However, once established, algae may become chlorine resistance in concentrations up to 5 PPM Free Chlorine.
The best algae prevention is prudent maintenance of free chlorine levels. Once algae is established you must treat the water with 30 PPM of Free Chlorine (Triple-Shock). This kills the algae AND destroys the organic waste left behind.

Algaecides can be used to make up for sloppy chlorine level maintenance.

There are 3 main types of Algaecides

1. Quats - Short for "Quaternary Ammonium Chloride Salts" or just "QAC" contain between 5% and 10% active ingredients. These are the least expensive algaecides but they usually cause foaming of the water.  This can especially be a big problem in a spa where the jets mix air and water.

2. PolyQuats - These usually contain between 30% and 60% active ingredients (Poly[oxyethylene(dimethyliminio) ethylene(dimethyliminio)ethylene]dichloride). PolyQuats are generally more effective and less foaming than Quats but they are much more expensive.

3. Copper Salts - These are very effective algaecides for ponds but SHOULD NOT be used in pools and spas because they stain surfaces.  READ THE FINE PRINT ON THESE PRODUCTS!