Hot tub electrical question.?

Sounds like your right on the edge of the maximum current draw and any little extra demand pushes you over the safe limit and because you trip both breaker and blow fuse its not a issue with either its a real honest overload so you need to isolate and measure with a clamp on type amp meter and compare to each spec for amp draw for both the pump and heater , you most likely have a motor with extra drag due to bad bearing or something or an intermittent short to ground somewhere