My hot water is hot enough but does not last long. On a gas water heater this usually indicates that the thermostat may mot be turning on the burner soon enough. You should hear the burner fire up on a gas water heater after running as little as 5 gallons of water. If you can fill your bathtub up more than 1/3 full with out the water heater firing up the thermostat may need to be changed. If your water heater is electric and your running out of hot water you probably have a lower element burned out. Diagnosing an electric water heater should only be done by a trained professional. Another cause could be that the hot water heater has lost capacity due to hard water deposits. Meaning that your 50 gal heater may only be a 30-gallon heater because there is 20 gallons of sediment built up inside the heater. If this is the case then replacement is the only option.