What could go wrong with a gas water heater? It heats water, and the thermostat seems to work. If we turn it up, it gets hotter, turn it down it is not as hot. But it seems that the tank might be only filling half way. Is this possible? What can be done, short of buying a new water heater>

What makes you think it is only filling half way? Is it just that you don't get as long a shower as before?

If so, then I'd start by draining the tank to remove sediment build-up. This can make a big difference in the hot water supply.

Essentially, what you need to do is to locate the drain valve near the bottom of most water heaters. You can connect a hose a plac a bucket under it. The water in the tank is hot, so you must turn down the thermostat on the water heater and then run the shower to drain off the hot water.

Now open the drain valve and drain off a couple gallons of water or until the water runs clear. Tightly shut off the drain, this can be tricky, because the valves are often low quality and so you must finesse them so they don't drip. Be sure to come back in a couple hours to make sure there is no puddle from a drip.

By draining off the sediment, you make room for more hot water and you make it easier fot the flame to heat the water. This will improve efficiency and hot water supply.

Let me know if this resolves your trouble.

We will try that. It certainly makes sense.