The following is a post from cichlidforums.com

by "abenez"

There is confusion about water clarifiers that needs to be "cleared up". There are three basic types: those that kill algae called algaecides, those that kill bacteria that cloud the water white, and those that clear out the particles suspended in the water _ the suspension looking cloudy brown or another color due to the color of the particles.

Green Cloudiness: When the water becomes clouded green, this means that single-celled algae have taken over, due to an excess of nutrients in the water and sufficient light for the algae's photosynthesis. There is no easy way to get rid of the cause for the algae, but regulating excess nutrient sources and light sources can help. Chemical algaecides, all regulated by the Environmental Protection Agency (EPA) are available on the market. Be careful in using these toxic products and follow their instructions carefully.

Particle Cloudiness: The clarifiers of water clouded by suspended organic or inanimate particles in the water are coagulants or flocculants that stick to the particles and drop them out of the water. They are then removed by filtration. The Kordon water clarifiers TransClear (fresh water) and Sea Clear (salt water) are in this category. They are inert and non-toxic.

White Cloudiness: New aquarium and pond set ups are sometimes prone to have excess nutrients in the water, usually from overfeeding the fish, or from dead fish or invertebrates. At such times gram-positive bacteria become so numerous in the water that they cloud the water whitish to milky white. While harmless to fish and invertebrates, they require a lot of oxygen and can suffocate them. Aquarium filters are mostly ineffective in removing them. The best way to get rid of these living bacteria is to find and remove the source of the pollution in the water. This is extremely important so that fish and other animals in the water are not killed by the pollutants. Once this is resolved, the bacteria, lacking a food source will disappear. The excess bacteria can be eliminated by using a bactericide that kills gram-positive bacteria, such as the antibiotic erythromycin.