When the water level in the toilet is too low, it could mean there's a clog in the toilet, the tank isn't filling completely or the vent is obstructed. You can correct all of these problems yourself, although in the case of a vent problem, it may mean climbing on the roof. Don't do that in rainy or snowy weather, and if your roof is steep, get help regardless of weather.
Proper Water Level
The toilet's internal trap regulates the water level in the bowl; it can't go any higher than the trap's upper curve and in normal circumstances, the two levels are the same. That part of the trap also is the one in which blockages are most likely to occur; if a partial blockage develops, it can siphon water out of the bowl when you aren't using the toilet. The blockage often is a feminine hygiene product or large wad of paper, and you usually can get it out with a plunger or a snake.
Use the Right Plunger
Unlike a sink plunger, which is cup shaped, a toilet plunger has a narrow opening that fits inside the toilet waste outlet. It's essential to use this type of plunger. To get the maximum cleaning power, fill it with water before putting it in the toilet to avoid pumping air, and give it several sharp pumps. If after a few days the water level still is going down, use another method to clear the blockage.
Snake the Obstruction or Pull It Out
A toilet auger, also known as a snake, features a corkscrew-like tip that digs into an obstruction when you crank the auger handle. Feed it into the waste opening and push until the tip hits an obstacle, then crank the handle. When you pull the auger out, the blockage should come with it. If you can feel an obstruction but the auger won't remove it, it may be something hard, such as a child's toy. If so, put on a rubber glove that covers your forearm and reach in to grab it. If you can't get it that way, you may have to drain the toilet, remove the bolts, lift the toilet, turn it on its side and reach in from the underside.
Adjusting the Tank Water Level
Low bowl level accompanied by poor flushing often is a sign the water level in the tank is too low. It's easy to determine if this is true; just lift the tank lid and look. Adjust the float to adjust the water level. If you have a ballcock float, screw the ball clockwise with respect to the armature to shorten the arm; this allows the fill valve to stay open longer. More modern toilets have a plastic fill valve you can adjust with a screwdriver. Turning the screw clockwise on the top of the armature shortens it, which in turn lifts the float and raises the water level in the tank.
Clearing the Vent
If the low water level is accompanied by slow flushing and gurgling in one or more sinks when you do flush, the rood vent probably is obstructed. This type of obstruction prevents air from getting in the pipes, and flowing water then creates a suction that pulls water out of the bowl. The roof vent is a short length of 2- or 3-inch pipe extending through the roof. Clear debris from the opening by hand and look inside -- you may find a bird's nest. If you don't see anything, spray water from a garden hose into the opening; an obstruction will cause it to back up, so keep spraying until the water empties freely.