Transporting coral fish is not always trouble free. If a single fish is moved while in a large tank of water, which is also filled with oxygen, as is usually practiced in specialized aquariological trade, then the entire move should be trouble free. But as soon as you want to transport a larger amount of fish things become more complicated, particularly during the transfer from one private aquarium to another, where instead of placing each single fish in a large tank of oxygenated water, they tend to group a number of different types of fish together in a plastic bucket.
The tank for transporting coral fish should have a large surface area in relation to the volume of water. However this is how the problems begin. The fish while giving off carbon dioxide and ammonium through their gills lower the amount of oxygen and the pH value of the water. Usually a high plastic bucket with a lid is used for this purpose. In addition there is a trend to fill it with as much water as possible, thinking that if there is more water then there will be more oxygen for the fish, or maybe not? In fact the opposite is true: the less water there is, the higher their chance of survival, at least to a point. This fact may seem initially like a paradox, but a tragic event from my early period as a marine aquarist, 20 years ago, should help to explain it. I wanted to carry 20 turquoise bridesmaids in a bucket of ten liters of water. I placed the fish inside and filled the aquarium with water within a couple of cm from the brim. After driving about three quarters of an hour by car, I arrived at my destination and opened the bucket. About two-thirds of the fish were dead and the others were splashing desperately around in the water looking for oxygen; but what had happened? Each fish had had a larger amount of water available compared to commercial exports from Indonesia or the Philippines while travelling to Europe (where, in fact, each example of fish has only a few tablespoons of water for transport). In my case each fish had had much more water so what was the problem? With a larger amount of water the metabolic products of fatty fish would have been diluted, or not? At this point it had to be absolutely an advantage to fill the bucket to the brim and not only to two-thirds or at least that was what I believed. However thing in life are often NOT what they seem!
The amount of water is not the decisive fact; in fact it is the relationship of its volume with the surface, which allows adequate gas exchange. For example: if you fill a 10 liter bucket with 9 liters of water a certain relationship is created between the volume and the surface with a passive gas exchange. The same amount of gas is passively exchanged through its spread on the water surface for each unit of fish and this gas exchange within 9 liters of water determines a particular level of oxygen and CO2 in the water. If the same bucket were filled with 1.8 liters instead of 9, the saturation of oxygen in the water would be five times higher! In fact, it is much more complex than that, because on one hand in a limited volume of water the metabolic fish products enrich more than a higher volume of water, on the other hand the higher concentration of these substances lead to a higher exchange of passive gases, this exchange of gases depends mostly on what is above the surface of the water, a sufficient amount of oxygen must be provided and in no case should carbon dioxide be allowed to accumulate, but must be allowed to escape, because otherwise it will hinder the exchange of gases. It becomes evident that with more water when transporting, with the same surface area in the container, problems are created not resolved. For this reason, today I always follow Prof. Ellen Thaler’s advice by always transporting coral fish in a reduced level of water. Even when it comes to a large surgeonfish, almost 8 cm in height, it’s better to transport in a bucket of 10cm of water instead of half or completely filled. Furthermore, the bucket shouldn’t be completely shut or at least it should be opened regularly so to allow movement to the water and to allow carbon dioxide accumulations to escape. The ideal situation would be to achieve also when transporting in a bucket, the same relationship between air and water as we find in small transport bags, in which coral fish are exported from their original countries. In this case there are only a few millimeters of water in comparison to the large surface area, above which there is a high supply of oxygen. This means, therefore, that a shallow tank is a better form of transport than a bucket. If you must use a bucket then you should fill it with less than two-thirds of water and open it regularly to help gas exchange. There are other ways to optimize the transport of coral fish.