The "Theory" Behind Ethanol Additions to Aquariums
A German magazine recently published an article suggesting and recommending the dosing of ethanol (as vodka) to reef tanks as a carbon source for marine heterotrophic bacteria in order to increase denitrification rates and bacterial biomass production (Mrutzek and Kokott 2004). Further, they claimed that additions cause rapid declines in nitrogen and phosphorus produced by fish, invertebrates, and algal metabolism (ironic, since many aquarium invertebrates and algae are sinks, not sources, for nitrogen and phosphorus). In turn, the bacteria provide a food source for corals and other filter feeders. The method is recommended particularly for those tanks that are highly skimmed (and probably lack particulate material) and which lack sand beds. Tanks with sand beds or other sediment-based systems, they mention, react unusually and may have adverse effects to ethanol additions.
"Experiments" were performed (and I use the term experiment loosely to mean the typical uncontrolled, unreplicated, statistically insignificant sort of "let's add it, see what happens, and produce results that show how my tank never looked better" sort of trials that are often found in aquarium literature). The results showed a precipitous decline in nitrogen and phosphorus levels over approximately one month with increasing doses of vodka. The sample size for the experimental procedure was one (n=1), consisting of a single person's personal home aquarium. There were no controls in the experiment (i.e. an identical tank without vodka being added to see if there actually were results from the treatment). In fact, the sample tank received an increasing dose of vodka during the treatment, making any dosing effect impossible to determine. Additional support for the "experiment" was collected by casual replication in completely different trials in even less controlled conditions; that is, other aquarists began adding vodka and claimed similar "results."
Results of this work also showed a number of other effects. A large "bloom" occurred which clouded the test tank, an occurrence that could and often does kill tank inhabitants. It was assumed the bloom was bacterial, but no mention was made if and how the cloudiness in the tanks was determined to be bacterial. Given what I will offer below, it may also have simply been carbonate precipitation brought about by additional carbon addition and possibly microbial mediation. Having fortuitously escaped tank mortalities, the tank cleared and the authors literally state how "the tank water had never been clearer, the coral polyp extension was better, and the coral coloration was more intense." Where have I heard this before? The observed decrease in nitrate and phosphate is an interesting effect.