What type of fluid condition requires closing the pool?

Closing the pool is necessary when the main drain is not visible. This condition poses a significant safety risk as it may indicate problems such as insufficient water clarity or debris accumulation, which can hide potential hazards or create an unsafe environment for swimmers. Visibility of the pool's main drain is crucial for ensuring that the pool is safe, as it helps prevent accidents such as suction entrapments, where swimmers can become stuck due to the strong currents generated by the drain.

The other options do not present the same level of immediate safety concern. A water temperature below 70° Fahrenheit may be uncomfortable for swimmers but does not necessarily warrant closing the pool. While a high number of bathers can lead to issues such as increased contamination, a well-managed pool should be able to handle this with appropriate chlorination and filtration. Normal filter operations indicate that the pool is functioning properly and does not require closure. Thus, the visibility of the main drain is critical for safety reasons, making it the correct condition that necessitates closing the pool.