BTW you said that you dose organic carbon to remove nitrate, but I thought this mechanism also needed phosphate to work. But you also use GFO which must take some phosphate out, it seems something is unbalanced there. Does that mean that your food probably contains more phosphate than your skimmate (assuming the end result of your carbon dosing is to make something which is skimmed out)? Or does your tank use the nitrate reduction method where nitrate is the energy source (but I thought carbon was not required for that one)?
Thanks, Pete
It definitely also takes out phosphate too, but is imbalanced toward nitrate because bacteria growing in low O2 regions use nitrate both as a source of oxygen (wasting the N in the nitrate, producing N2) and also as a source of N for body tissues.
The GFO would ideally take up the left over phosphate.
Here's a blurb on organic carbon dosing from an upcoming nitrate article:
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 O
2 regions (such as in sand or rock) and partly in highly oxygenated environments. Since metabolism in low O
2 regions uses relatively more nitrate than phosphate compared to metabolism in a high O
2 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 O
2 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).
