o yes, I've kept so many goldfish before and they often met their demise in my overpowered filter like that. the remedy is to somehow put a sponge pre-filter on your filter intake. still if your tank is too small, nothing can help your fish but more space. for two fancy goldfish a 30 gal tank is minimum.
you can forget about keeping comet and common goldfish in regular small tanks, those need ponds.