I bought a used water heater from a guy on craigslist about 2 months ago for a new, new to me, house I bought. The water heater is only a couple of years old and is a gas heater, and a 50 gal tank. The problem is it seems to only get about 10-20 gallons of hot water and then cools down after that. If I fill one of the tubs of with only hot water, the tub only fills up a few inches of hot water then then turns cold. THe hot water is hot, very hot. Maybe the tank is only partly getting hot? Is there something I could check? I don't know anything about heaters but I was told with electric it possibly could be a heating element going out but I'm not sure what happens with gas and what could be causing this.