@Meyer Jordan Is a high level of phosphate bad for the fish and other pond life? Or is it just unwanted because it encourages the growth of algae?
Phosphorus is directly toxic to animal life only at extremely high levels. It is the response of plants and algae to elevated levels that is a major water quality issue.
"Phosphorus can be toxic, but toxicity occurs rarely in nature and is generally not a concern. Of more concern are the indirect effects of phosphorus. All algae and plants require phosphorus to grow.
Elevated phosphorus levels, however, can increase a freshwater system’s productivity and result in large amounts of organic matter falling to the bottom. Bacteria and other organisms decompose this matter and in the process use a lot of oxygen. In very productive freshwater systems, the oxygen levels can be in such short supply that fish kills occur.
A type of algae, called cyanobacteria, grows particularly well in high levels of phosphorus. Cyanobacterial blooms can cause a range of water quality problems, including summer fish kills, bad odours, and tainted drinking water. Some cyanobacteria produce toxins that can kill livestock and wildlife."
National Guidelines and Standards Office Environment Canada
It is important to point out that cyanobacteria blooms in garden ponds is extremely rare.
One thing that this link does not mention is that the proliferation of algae growth can effectively block sunlight from reaching the lower levels of a pond resulting in algae at these levels dying off. Bacteria activity will increase due to decomposition of this dead algae consuming even more Oxygen from the water column.
Effective aeration may help, but the source of the Phosphorus should be eliminated or a chronic state of imbalance will exist.
