Every aquarist knows the result of a well performed skimming operation: a smell “soup” of an olive color contained in the collection cup. This liquid consists of a solution comprising of many different substances, all of them removed from the aquarium’s cycle. But a good skimmer will not always make us happy, since in the recipient we will have the decomposition in concentrated form, through which a cessation of odors will happen. This mainly involves proteins, usually made up by amino acids containing sulfur like methionine, cysteine and homocysteine.
These are useful to bacteria, which have also been skimmed away from the aquarium, as nourishment and they are decomposed in smaller particles. During this process the bacteria will use so much oxygen to the extent of rapidly forming an anaerobic environment inside the collection cup. Inside the container, chemical processes will take place during which bacteria will extract energy from sulfur loaded compounds, reducing them to gaseous hydrogen sulfide (H2S). It is a toxic gas, one that can be appreciated as a “malodourous bomb” even with a dilution of 500,000 times. A reduced quantity can melt inside the liquid contained in the foam collection cup, but if the latter starts to build up, the former will eventually spread to the surrounding environment, making itself noticeable. Such an eventuality can be avoided anyway. To this purpose, bacterial activity has to be inhibited. What could we introduce inside the foam cup in order to make it non-toxic for the aquarium? This feature is very important, because it is possible that part of the skimmed material may end up in the aquarium despite all of the precautions previously taken. The solution is very simple, every aquariophile knows about “calcareous water” (lime water), a calcium-hydroxide based liquid able to increment the calcium content of sea water. Calcium hydroxide in water is a medium alkaline substance able to increment the pH value. An elevated pH value though can prove deadly for most bacteria (and for other living beings) or at least notably inhibit their metabolism. Hydrogen sulfide already contained in water reacts and is melted fairly well in this alkaline environment and it will no longer come out in gaseous form. Since proteins will no longer withstand further decomposition processes, ammonia too will not form anymore. In order to increment the pH value in such a reduced volume in a sufficient way, a few drops of a calcium hydroxide saturated solution will be enough. This quantity, if confronted to that of calcareous water dosed in the aquarium, is very reduced.
In case of accidental spilling of the content inside the skimming cup of the aquarium, there will not be any danger because of the calcium hydroxide. I personally dose the calcium hydroxide saturated solution in a quantity of one or two drops every ten milliliters of skimmed fluid. A bigger quantity is not dangerous, and probably a minor dosage can be sufficient. It always partly depends on the content’s composition. In every case, bad smell is beaten!