i remember someone said that if the DBS is not a wide as the main tank it won't be efficient enuf....u might wanna have it in your main tank as well as in sump....
as for your low nitrate now, it might be due to your H&S....basically the equation works this way,
dissolved organic compound > ammonia > nitrite > nitrate
so if DOC is low, theorethically, nitrate will be low too....so i suggest u might wanna invest in a SUPER HUGE DUAL BECKETT to skim out watever DOC that is present in your tank....
although H&S is a powerful skimmer, wat i heard is that it has low turnover rate, therefore there might be a danger in tank crash....the skimmer is not able to cope with the high die-offs....
remember to use netting to seperate the bottom 3-4inches of your DSB from the top layer so that there wont' be any disturbance....