pH stands for potential of hydrogen.  The pH scale represent the relation of hydrogen ions to hydroxyl ions.  Higher hydrogen content equals more acidic water, and as hydroxyl ions outnumber hydrogen ions, the water becomes more basic.

Typical ponds have a pH range of 6 to 11, which is slightly acidic to strongly basic.  For instance, 8.2 would be a very acceptable pH.

Bacteria is very important in the function of an ecosystem, and there are countless types of bacteria, something for every function.  Bacteria prefer to be anchored to a substance like rock or gravel, and are found in largest quantities in the midst of decomposing pond scum.

Bacteria work together to form a giant recycling plant be taking waste and dead material and converting it back into usable nutrients in the food chain. 

Tip:  Putting rocks and gravel in a pond significantly reduces the amount of muck buildup in the bottom of the pond.  This is because rocks and gravel provide many places for bacteria to live, much more than bare liner. More bacteria means more capacity to break down fish waste and plant debris, which leads to better water quality.

Low oxygen levels can result in the sudden death of an entire fish population as well as seriously impair the effectiveness of bio-filters.  Additional aeration of the water should be provided (get a bigger pump).  In a very small pool, a simple aquarium air pump fitted with an air stone or diffuser bar will suffice.  A larger pool may require an electric air compressor to pump air. 

Remember during the winter is when this most often happens.  If, you let the pond freeze over.  During the winter, use a pond heater to keep a hole in the ice.

Also to have a algae clean pond you will need to use live bacteria as talked about in the bacteria section of this page.

This will not rid your pond of algae, or keep it from getting an algae bloom, but this will help your pond cleare up faster, and stay clear.