Levels exceeding a threshold of 50 parts-per-million of cyanuric acid can reduce the effectiveness of the chlorine in a pool. This lowers the amount of bacteria a given amount of chlorine can clean up.
Cyanuric acid is a substance used as a stabilizer, mixed into pools to keep the chlorine from being lost due to exposure to sunlight over time. It protects chlorine atoms by forming a weak molecular bond with the chlorine, which keeps the chlorine from forming other bonds. While this does protect the chlorine from the decay that ultraviolet radiation causes, it also reduces the ability for chlorine to react to the bacteria or other microbes in the pool, reducing its ability to fight them.
The most effective amount of cyanuric acid that can be used without ruining the germ-killing strength of the chlorine is about 20 to 50 parts-per-million. Above this amount, the cyanuric acid levels begin to impede the effectiveness of the chlorine too much to be worth it.
Putting too much cyanuric acid into a pool could be an easy mistake, as cyanuric acid does not break down the same way that chlorine does. The levels of cyanuric acid in pool water remain fairly consistent for a long time, so removing it could mean replacing the pool water with fresh water.