I replaced all parts and the ele. Breaker keeps tripping

A few things come to mind.
Did you turn the breaker on before the tank fillede?
Heater elements take 19 amps and operate one at a time. It's possible that you wired the thermostats so both elements came on at the same time drawing 38 amps.
That would blow a 30 amp breaker every time the states called for heat.
Check your wiring. Good luck. Tom