Waited 6 weeks for an answer on this one. We were told that the reason, the water level in our toilet keeps going down to like 3 inches after it is flushed, is because our air vent is blocked. We live in a Mobile home, and can not find one air vent going thru the roof and only one under the sink in kitchen. Are they in the walls? This is a regular toilet, about 15 years old, works great, except within an hour or less, the water level is way down.
It sounds like to me, if the toilet is flushing fine, that your flapper or even your flush valve in the toilet needs to be replaced. You can buy a kit from Home Depot, and probably else where, which contains everything you need to do this for about 18 bucks. I would replace all of the guts inside the toilet. If the flapper is leaking this bad, you should be able to hear the water dripping past the flapper, and the toilet should always be filling up, some people say “it sounds like it flushes by itself” or “it keeps me up all night”.