I've been running Vodka/MB7 dosing recently. I've been reefing for 9 years and most of that time has been spent in SPS. In my history with SPS I have only dosed vodka twice, and both times were temporary in order to get nutrients under control.
I've had great success with ULNS but like others have said you're riding a tight line, and the slightest deviation can lead to catastrophic failure. I just recently went through one of these such failures. ULNS can be successful but it requires a LOT of time and, IMO, top notch equipment.
I tend to overstock my tanks. So, I do like to vodka dose when my system is heavily stocked. If you are going to vodka dose it is imperative that you have an oversized skimmer. Vodka dosing works by allowing an organic food source for nitrifying bacteria. The increased nitrifying bacteria then theoretically should process more phosphate and nitrogen compounds as their population continues to grow. Eventually the bulk of your systems phosphate and nitrogen compounds are present in bacterial biomass which your skimmer would remove.
So, if your skimmer is undersized you won't efficiently be able to remove the phosphate/nitrogen loaded bacterial masses. Secondly, carbon dosing can lead to oxygen snuffing. So an oversized skimmer helps supplement any depleted oxygen.
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