When flushing the commode tank fills, then water level falls and water begins to slowly flow again. Flapper has been replaced. Where else do I check for the water leak?

To check the flapper or the seat for leak, close the angle valve under the toilet tank after a few minutes lift the tank lid see if the water level is down if it is then you have problem with your flapper, the seat or the chain is too tight it keeps the flapper up not seated properly.

First check the linkage between the flapper and the flush lever should have 1/4" play when the tank is filled if not adjust the linkage.

If above is OK though you have changed the flapper, feel around the flapper flat edge. If it's wavy or you feel rough spots replace the flapper again, if the flapper is OK, clean the seat if it still leaks replace the flush valve see the image below, also check the small tube coming out of the top of the ballcock and make sure it's connected to the white overflow tube and draining water in to the tube, Good luck.

John

flapper has been replaced. When you replaced the flapper did you cut off the ring? (see image) If not do so and set the linkage so it has a quarter inch of play when the flapper's seated. Good luck, Tom