''People with Deep Sand Beds (DSBs) have reported issues with vodka dosing. These issues seem to be from increasing vodka additions over time without seeing a decrease in nitrate/phosphate levels. Users with DSBs that are failing to reduce nutrients efficiently may be experiencing a lack of nutrient decrease from the build up of organics within the DSB. If this were the case it would take more time to cycle out the nitrates within the system as there is now a hidden bed of organic waste that needs to be removed. Do not get discouraged or increase your dose irrationally as the vodka addition is working but not detectable as the DSB needs to cycle through completely before a reduction is observed. Keep adding vodka slowly until decreasing levels are detected. This may take months to achieve. Once there maintain that dose until nitrates start to decrease. At this point, it may be best to cut the dose in half and cut it in half again once undetectable is reached. Many people with DSBs who have reported issues increase vodka additions quickly and irrationally, leading to a detrimental overdose."
I just copied this from the reefkeeping article on vodka dosing, maybe it'll help.
