I don't kill anything, the whole system is in operation when the timer kicks on and starts the w/c
still tweaking and it's only on my 125 currently but I don't see a need to shutoff the pump or stop the overflow. my overflow is already faster than my pump and the incoming water is cut down to merely a trickle so there's no way the tank would overflow. then the overflow pipes in the sump drain to the yard.
still haven't hooked it to the pond as I'm currently digging the pond out again after a hedgehog/gopher/mole/??? fiasco.
on the 100g, I shutoff flow to the tank via a valve from the pump and open up a valve to a line that goes outside, the pump then starts draining the sump while the overflows continue draining until they run out of water level.
then I fill it back up and flip the valve's back.