Who pays for water and heat? As the owner, don't you have the right to inspect the apartment and enter to make necessary repairs?

You know he's gone; you hear water running; you may enter the apartment. Leave him written notification of the date, time and reason. Since he indicated there was a leak in the toilet - there is your reason to enter. Take a video camera, start filming before you enter, use a newspaper or other means to verify the date and time and film what you find.

sheelah-
Get over to that apartment and investigate pronto! If they are using 3400 cubic feet a week that translates to over 2.5 gallons per minute. No leaking toilet would use anywhere near that much. To use 2.5 gpm, you would need to have one or two shower heads on continuously at full blast. Your supposition that he left the water on just to be a jerk is probably right. He's running up a huge water bill on you...maybe $150-200 a week.