in closed systems, where filters dont get oxygen the anaerobic bacteria kick in and
their "product" is H2S, hydrogen sulfide which creates the stinky smell of fetid
ponds. this product is extremely toxic to fish and when the pump is started again
the built up H2S is dumped into the pond.

I agree. split the pumps and run a highly efficient one on the filters all the time,
OR, switch to a shallow veggie filter where anaerobic conditions wont occur, Ingrid