Leak from Flush seal

I have a one year old Kohler low flush toilet that started leaking from the bottom of the tank. I ordered a new seal for the flapper assembly and now when I tighten the nut on the bottom and put water in the tank it leaks like a sieve. Am I tightening the nut to much or is there sealer that I need to put on the seal ring?

Just as a thought. I am using a large pair of pliers to tighten the large nut under the tank. I am thinking that I am squeezing the nut on to the threads which is causing the nut to bind and not tighten all the way. Is there a special nut tightener for this??