Does running AC motors at lower voltage do anything bad?

I am using a Variac (variable transformer which reduces the applied AC voltage) to reduce the speed/output of an AC pump (aquarium/pond type).

Does this have an adverse effect on the pump in terms of:
life expectancy ?
heat generated ?

be cool

Good question (deserves a bump)

My ceiling fan doesn't seem to mind the speed control, and it's 10 years old, though a fan is fan cooled (dig that crazy logic).

No, it won't damage it. However, obviously if the voltage is too low then it won't even start.

Running things slower than they are designed to always makes them last longer.

That's just MHO.

No should not hurt the motor part of the pump I have seen this done many times in the mining industry for soft starting long belt conveyors it should produce less heat at the lower voltage unless I have my thinking backwards In any case it should not produce more heat. Life of the pump and such I cant think of why it would hurt it but I realy dont know.