| I am assuming that by volume you mean that you are not getting fifty gallons of hot water out of your water heater. If this is what you meant I will as you a question, have you been finding little white things in the aerators on your faucet spouts?
The applies only if you have been finding those little white things which are fine pieces of plastic. Your water heater has a tube in the cold water side of the heater, it is there to take the cold water entering the heater to the bottom of the tank, therefore not mixing with the hot water that is in the top of the tank.
The way to check this is to that the cold water loose from the heater, and depending what brand of water heater you have you look for this dip tube (which is the name for it). some heaters you have to unscrew the pipe out of the heater, others you just have to lift it out of the nipple that is screwed into the heater. If you pull it out and it is about four foot long then that is not your problem, If it is only a few inches long you have found the problem and it can be replaced with a new one from a hardware store.
Let me know if this was your propblem. | 
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