My hot water heater is over heating and tripping the breaker on the thermostat. I replaced the thermostat with no difference. The hot water heater is 8 years old and never been flushed until yesterday and what a mess, but it still it did not help. Any ideas

If you've replaced the thermostats and it's still blowing the breakers the next step would be to replace both eleiments. If you have access to a AmpProbe the heater should not pull any more then 19 amps.
Be advised that these are opposing elements. Only one on at a time.
If the thermostats are wired so they both come on at the same time this will overload the breaker.
Double check the thermostat connections and if they're correct replace the elements. Good luck, Tom

I should have mentioned its only a single element hot water heater. I thought about changing the element, but it seems if the thermostat is working correctly that shouldn't matter.

What do you mean that the thermostat seems to be working correctly if the heater is over heating.

If your element is shorted out to ground on the leg that is not controlled by the thermostat it will continue to heat until it trips the reset button. So I say you change the element and see if that solves the problem

I never thought about the element shorting to ground.

Thanks I'll change the element and see what happens.