if u dun cycle with your sump, ensure tat there is enuf water circulation in your sump....sea water is not sterile, so not running it will make it turn foul....like rotting food.....and by not running both together, there will be a slight risk of not having a balance in the water parameters of the tank and the sump....this should be easily corrected by running the sump for a few days prior to adding any LS....