[Scuba Dog]

could be that you already have super low nutrients in your tank which is an exclent thing, do you have any nunsance algae in the display as in hair alge or some cyno? if any of those two are present then you have some nutrients in the water...one thing you can do if your running a Ca Rx is to have the effulent drip in to the fuge that way the macros will bennifit by the trace amounts of Co2...