How do I get a toilet tank to stop leaking between the tank and the comode?

I just replaced the wax seal on the comode and changed all the plumbing inside the tank. I put it all back together and it leaks when I flush it. As long as the stop is in place, it will not leak, but the second I flush it, water comes out from where the tank meets the comode. I don't want to tighten it much more as it might break the tank. Help