Toilet bowl water level is set by the internal trap geometry and by how much water the refill tube returns after a flush. A low level can result from a misplaced refill tube, incomplete refill, siphoning related to venting, or a crack that slowly loses water.
See whether the bowl starts low immediately after a flush
If the bowl never reaches its usual level, inspect the small refill tube inside the tank. It should direct water into the overflow tube as designed rather than hanging loose elsewhere.
Check tank water level as well
A tank that fills too low can reduce both flush strength and bowl refill. Check the marked water level and the accessible fill-valve setting before adjusting other parts.
Water-level changes without flushing can point to venting
If bowl water drops when a tub or washer drains and gurgling occurs, the system may be siphoning because of restricted venting or drainage.
Slow unexplained loss may require inspecting the bowl
A hairline crack in porcelain can lose water in a way that is not immediately visible and is not reliably repaired with sealant. If you see a crack, moisture at the base, or movement of the bowl, stop using the toilet until it is assessed.
Sources
- U.S. Environmental Protection Agency WaterSense — Home Maintenance — Household maintenance related to water, leaks, toilets, pressure, and efficient use.