If you use drop-in toilet fresheners/cleaners in the tank, they can cause the flapper and other parts to fail. They also will void the warranty on a new toilet. I would take the advice above and replace the entire guts of the toilet with the same brand. Also add Rid-X and/or buttermilk once every 2 months to help the septic system. More if you use alot of bleach when you wash clothes.

 

There is a flapper on the market that is adjustable with an adjustment on the bottom of the flapper that you can click to widen and get a great seal, try one. If not just go get a new toliet they are fairly cheap to replace. I have had my septic for over 10 years and have never had a problem with it.

 

Frank S makes a good point. I can't believe I neglected to mention that. Thoes in the tank cleaners wreak havoc on all the different materials in the tank. Especially the different grades of rubber flappers are made of. If the toilet is an American Standard or Gerber and there is an in the tank cleaner in there the flapper wont stand a chance. I don't know about the other brands but the Kohler toilets have a sticker in the tank that says evidence of in the tank cleaner will void the warranty.

Think about your septic system. It's designed to accept water and human waste. If the leach fields are working properly then it shouldn't matter how much water you send through it, with in reason (the system is designed for a certian usage). The solids take a while to break down so we have the tank pumped around every three years, depending on how many people are in the house. If you haven't had the tank pumped on a regular basis, your leach fields may have too much effluent in it. If the property was perked properly and the leach fields designed properly a toilet running shouldn't cause the system to fail.

Another thing that can effect a septic system is discharge from a water softener system. The salt and/or brine can kill bacteria and render the septic system useless.