Relation between NO3 & Flow/Flow Direction

[Kevotron]

My NO3 use to be really high and now that i have been using the Zeovit system for a few months
is lower.. @25mg/L (Elos test kit)
nothing seems to be stressing (except Plate Coral) SPS has decent growth & color..
however 25mg/L still is not desireble I want it much lower..
so now im questioning my flow or direction of the flow in the tank is not good enough for NO3 to be efficiently rid of by my sump/zeovit reactor
Can anybody offer some advice ??