Hi, the water in my toilet tank continually runs and only fills up halfway. When I turn the water off, the tank drains completely. Right now I have it turned completely off, I have no idea where that water is going. What could be wrong? Thanks...

Most likely the flapper is leaking. Easy fix. Take the old one off and get a replacement. Some brands require a specific flapper so make sure to match it up.

Agree and you will confirm, if you'd like, by dropping some food color in the tank. Chances are your bowl will show color in moments. Check the chain to the flapper, it must have slack and sometimes they get twisted up and hold the flapper off the seat.