My tenant says in the morning the toilet tank is empty. After flushing it refills. Apparently it is fine until the next morning. What could be the problem?

For the tank to be empty you need two problems at the same time. The only way for the tank to leak down is a bad flapper. However, you would also have to have a bad ball cock which should be refilling the tank when it gets low.

Which type of toilet does he have?

https://tse1.mm.bing.net/th?&id=HN.608005728483804551&w=300&h=300&c=0&pid=1.9&rs=0&p=0

After flushing it refills
If the tank is empty how can he flush?

I am going to take wild shot at this and suggest it may be the bowl that is empty, not the tank. A bowl can be siphoned dues to a clogged vent stack but you can still flush it each morning.

There are at least two more ways for tanks to empty. A re - fill pipe, that is stuffed to far down the overflow tube, can siphon water from the tank; this pipe should be aimed into the overflow with an air gap between it and the top of the overflow tube. The rubber flush valve gasket can soften /erode, often due to high concentrations of chlorine. A failed flush valve gasket / connection can leak small amounts of water from the tank to the bowl - enough to empty the tank overnight, however, as mentioned by ballengerb, there may be a problem with the ballcock if the tank is not refilling...

And another obvious answer, if it is indeed the bowl, not the tank, and the tenants have dogs, the dogs are drinking the stinking water, lol, happens every day at my house.

Where you been MGD? Welcome back.

Yep MGD we been worry about you welcome back, Thanks.

John

Just been extremely busy with the business. Things are going good, stressful, but good. Good to see you guys still around. Lee.