Answer:
Increased levels of phosphate are beneficial to phytoplankton and aquatic plants. In the short-run, this means increased micronutrients for herbivorous and omnivorous fish.
The phosphate, however, reacts with the water to reduce the oxygen levels obtainable in the water. Without oxygen, the fishes die off.
It is not unusual to find that with increased phosphate in water, healthy fishes begin to die off without any other plausible explanation.
The solution is to clean the water or treat it with solutions that impact positively whilst reducing the phosphate levels.
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