I am considering the use of biopellets as my carbon dosing method... is that advisable or is there a reason vodka dosing is superior? Also is it worth buying a $350 recirculating reactor when a Two little Fishies reactor is $30? In addition, I'm assuming if one is using biopellets and some sort of GFO/GAC one doesn't really need a deep sand bed or chaeto?
Both can work well, and both have drawbacks.
A drawback to pellets is that it is not as easy to alter the "dose" over time as needed.
A drawback to soluble products is perhaps more potential for cyano.
I personally like the fact that many organisms can use the acetate in vinegar, corals, included, so it is not just a way to drive bacteria.
I compare them (and other methods) here:
Nitrate in the Reef Aquarium - REEFEDITION
http://www.reefedition.com/nitrate-in-the-reef-aquarium/
Biopellets
Biopellets are typically organic polymers (often PHB, polyhydroxybutyrate; shown below)):
Polyhydroxybutyrate
These polymers can very slowly degrade in sterile water, but in an aquarium, bacteria settle on them and release enzymes that chop up the polymers into smaller bits (such as hydroxybutyric acid, the building block of this polymer) much faster than the simple hydrolysis reaction would proceed on its own. PHB is a natural energy storage molecule for many bacteria, and so some bacteria are accustomed to making it and degrading it internally. Some bacteria earn a living by being ready to degrade such polymers when they encounter them outside their bodies (for example, from other bacteria that have died) and these bacteria release enzymes into the water to break down the polymer pellet. They then take up the released hydroxybutyric acid and metabolize it to gain energy.
In an aquarium setting, bacteria will coat the pellets, and digest much of the polymer themselves, using up nitrate and phosphate in the process as they grow and expand their tissues. Some of the released hydroxybutyric acid will make its way into the bulk water, so bacteria can also grow remotely from the pellets (including cyanobacteria). Consequently, it is not as confined of a process as is a carbon denitrator, even though it is usually carried out in a flow reactor.
This method can use slightly more nitrate than a proportional amount of phosphate if the bacteria form a thick enough layer on the pellet. In that case, the bacteria on the bottom (the pellet surface) can be oxygen limited, and may use nitrate as an electron acceptor (source of oxygen) as happens in deep sand beds, in addition to the N and P used to make their body tissues.
Organic Carbon Dosing
Organic carbon dosing involves adding a soluble organic compound to the aquarium which spurs bacterial growth. Typical organics used can be ethanol (as vodka), acetic acid (as vinegar), calcium acetate (as lime saturated vinegar), sugar (sucrose) and many others. Vodka and vinegar are by far the most popular. I use vinegar.
These organic molecules can be used by many organisms, including corals, but the main intent is to drive bacterial growth. To grow, the bacteria need a source of nitrogen and a source of phosphate, and a large portion of these they remove directly from the water. The bacteria may grow out of sight (inside live rock or sand, in refugia, in tubing, etc.). They may also grow in globs in the display tank. They have to grow somewhere. If they become unsightly, try dosing a different organic that may drive a different set of species that may grow in a different location. I’ve had them often seem to grow on GAC (granular activated carbon media) in a canister filter I previously used, allowing relatively easy export by rinsing the GAC once every couple of weeks.
I’ve never heard any plausible argument why dosing multiple organics at once is desirable, but many people do it and there is likely no harm in doing so. The idea that multiple organics drive a diversity of bacterial species is just speculation, and even if true, I don’t see the benefit.
The bacteria themselves can then be skimmed out, or used as a food for filter feeders, or both (most people probably have both to some extent, unless they do not use a skimmer). The bacteria may grow partly in low O2 regions (such as in sand or rock) and partly in highly oxygenated environments. Since metabolism in low O2 regions uses relatively more nitrate than phosphate compared to metabolism in a high O2 environment, the relative amounts of nitrate and phosphate reduction an aquarists observes may vary from system to system.
Nitrate is always reduced to a greater extent than phosphate simply because bacteria need a lot more nitrogen than phosphorus, but metabolism of organics in low O2 regions may skew it even more, and sometimes can leave the aquarium with little nitrate and an excess of phosphate that they bacteria don’t “want”. In such a case, a phosphate binder might usefully export this remaining phosphate. Alternatively, some aquarists have dosed nitrate directly to the aquarium to allow the residual phosphate to be consumed.
These linked articles describe
vinegar and
vodka dosing in more detail.
One potential drawback that may have played a role in some tank problems is that the bacteria that thrive when organic molecules are dosed may be benign (and appear to be in almost all cases), but might actually be pathogenic in others. That is, the added organics may enhance bacterial infections if those bacteria causing the infection (of fish, corals, etc.) are able to take up the added organics and use them to grow faster. I think this risk is low, but it may be real. If you have unexplained problems that might fit this description, and are organic carbon dosing, try not dosing for an extended period.
A second potential drawback of organic carbon dosing is the potential for proliferation of unsightly cyanobacteria in the display tank. There are many species of cyanobacteria, and some can consume the organics we add in this method. If they become a primary consumer, then something may need to be done, such as switching to a different organic compound to dose, or reducing phosphate with a binder such as GFO (granular ferric oxide).
