OUr 5 yr old duravit toilet is constantly running. Seems that tank water is seeping into bowl causing the mechanism to demand more water. Onl way to stop is to turn of water valve until we need. Before I call a plumber is this fixable?

If you can see the water running in the bowl it's a good chance that the flappers warn http://www.aplumbers.com/residential/toilet
1. tank the tank lid off.2. flush the rubber thing that comes up and allows the water to go to the bowl is the flapper 3. remove 4. replace that should fix it.
problem 2
1. tank tank lid off.2. if the waters flowing over the long tube in the center and down then the water level is too high3. adjust the level either it will have a ball **** or a float in either case   a ball **** adjusts by a screw on the fill valve. the float has a either a metal rod w/ a pinch clamp or a plastic long screw type thing.4. adjust water level .5. barely lift handle to allow a little water out til fill valve kicks on then release handle til valve stops .6. just keep re adjusting til you get the water level right there should be a molded in line for the level on the back of the tank.