The primary drawback to sulfur denitrators, IMO, is that it depletes alkalinity, and it does so in a way that depends on the nitrate level in the tank, so is not always the same.
I discuss them here:
Nitrate in the Reef Aquarium - REEFEDITION
http://www.reefedition.com/nitrate-in-the-reef-aquarium/
8. Sulfur Denitrators
In these systems, bacteria use elemental sulfur and produce N2 from the sulfur and nitrate according the following equation (or something similar):
2 H2O + 5 S + 6 NO3– → 3 N2 + 5 SO42- + 4 H+
The production of acid (H+) in this reactor can tend to reduce the aquarium alkalinity. It has also been suggested to pass the effluent of such a reactor through a bed of aragonite to use the acid (H+) produced to dissolve the calcium carbonate, and thereby provide calcium and alkalinity to the aquarium. While that is a fine idea, it doesn’t add much calcium and alkalinity to most aquaria.
To estimate the magnitude of the effect, we start with a liberal estimate of how much nitrate might be removed. Say 10 ppm of nitrate per week.
10 ppm nitrate = 0.16 mmole/L of nitrate
Since 4 moles of H+ are produced for every 6 moles of nitrate consumed, this will produce
0.107 mmoles/L of H+ per week
How much calcium this could produce?
Assume that it takes one proton to dissolve one calcium carbonate:
CaCO3 + H+ → Ca2+ + HCO3–
Clearly, this is a substantial overestimate because much of the acid will be used up driving the pH down to the point where CaCO3 can even begin to dissolve. Consequently, we have an upside limit of 0.107 mmoles of Ca2+ per week. Since calcium weighs 40 mg/mmol, that’s 4.3 ppm Ca2+ per week.
For comparison, an aquarist adding 2% of the tank volume in saturated limewater daily is adding on the order of 16 ppm of calcium per day. Consequently, this method may not be especially useful for maintaining calcium. Additionally, the acid produced will have a long term lowering effect on the alkalinity. In fact, it is double dipping on the alkalinity depletion since alkalinity is consumed when the nitrate is produced, and again when it is removed in the denitrator. So if you use a sulfur denitrator, be sure to monitor the alkalinity in the aquarium.
