Recently I have noticed the water level in the upstairs toilet is much lower than it has been. The bathroom is back to back with another bathroom and located over the kitchen. I have monitored the water level with the other fixtures using water and without using water and the result is the same. After flushing the toilet the water level is full. Over several hours the water level in the bowl decreases sometimes to the point of almost being empty. The toilet has been plunged. The tub and sink drains have been plunged and treated with a drain cleaner to eliminate the possibility of a clog. What is happening and how can I correct it? Thanks for any ideas and help.

 

Most likely a vent problem. But you can conduct an experiment.

Flush the problem toilet so that the water level is normal. Then without using any other water in the house, take the whole family to a movie. When you return (with no water having been used since you left), check the water level in the bowl again.

If the water level stays the same with no water usage, then you have a vent problem. Go up on your roof and run a snake down the vent stack serving this toilet. Perhaps some bird built a nest on top of your vent, or leaves from a nearby tree have clogged it.

If the water level drops even with no other usage, then it's probably not a vent problem. In this case, you could have (1) a cracked toilet bowl leaking water down the drain, or (2) a foreign object in the trap that is wicking water from the toilet. Both of these will require you to pull up the toilet. If you've never done it, it's not as hard as you might think. Just ask here and somebody will give you instructions.