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10-20-2008, 08:35 PM #1
Dispelling Some Myths about Adding Salt to FW Aquariums
Recently there have been a number of threads arise regarding the use of salt in freshwater aquariums. Unfortunately, there are some myths out there regarding the use of salt and exactly what it will do. There are still many that use salt and sing its praises, it is just one more thing that does not need to be used all the time and I'd like to clear up a few of the myths surrounding it. The following statements are not merely the opinion of this hobbyist, they are founded in scientific fact.
1. The first idea that I'd like to deal with is the ideal of "Relief of Osmotic stress." This is an idea that is not entirely a myth, however it is often presented as a blanket statement and that is where the myth arrives. For fish that come from environments that have 0 salt (aka, most captive bred fish) the addition of salt does nothing to relieve osmotic stress, it creates it. For wild caught fish that come from brackish environments or environments where there a trace amount of salt can be found, the addition of a very small amount of salt will help with osmotic stress.
2. Myth number 2 deals specifically with livebearers. Mollies, Platies, Swordtails, and Guppies are often presented as being brackish fish or fish that require salt. This is a completely false idea. Platies, Swordtails, and Guppies are all completely freshwater species. Mollies are not a brackish fish, they are however a euryhaline species. This simply means they can adapt to a wide range of salinities. They key word there is adapt, they do not require salt at all but they can adapt to levels ranging from none all the way to full marine.
3 and 4. The third myth deals with disease prevention and the fourth does right along with it; stress reduction. Again, this is not entirely false but it is still entirely unecessary. Salt allows the fish keeper to keep lower than ideal water quality and it reduces the chances of diesase as a result. However, the idea that it will prevent Ich is not true at all. The logic seems to be, since salt kills free swimming Ich, it will keep it from forming. This is not true, and it has in fact lead to a strain of Ich that is entirely resistant to salt. Ich is a parasite and a very adaptable one at that. Maintaining salt in your system all the time simply allows the Ich to adapt to it and develop an immunity to it. Also in this area, the logic seems to be that salt helps create a thicker slime coat which in turn makes them more disease resistant. This reason has to be the most pointless of all the reasons for using salt. While it is true salt does cause a fish to develop a thicker slimecoat, the development of the thicker slimecoat is a reaction to an irritant in the water, and that irritant is the salt. Irritants in the water are a source of stress, and stress is a cause of disease in fish. So, here is what the addition of salt boils down to in this case: salt irritates fish and creates stress -> fish develops thicker slimecoat in reaction to irritant -> fish becomes more resistant to diesase that results from a stressful situation. At this point, the fishkeeper would be just as far ahead without the addition of salt, because with out the salt, there would be no creation of a stressful situation that the fish needed a defense against. Completely self defeating reason.
5. Another reason some people site for using salt is the addition of electrolytes to the water. While this may be true, the exact same thing is accomplished by simply doing a water change. Again, the addition of salt is a redundant and pointless act in this case. The salt concentration of fish blood is 15-17 parts per thousand. In freshwater the surrounding environment has a much lower salt concentration and therefore the tendency for equilibrium causes the water to diffuse into the bloodstream through the thin gill walls through osmosis. As you raise the salinity of the surrounding environment the rate of osmosis slows down. When you reach concentrations that are equal, osmosis completely stops because equilibrium has been reached. That cessation or slowing down of osmosis slows down the electrolyte intake as well. So, the introduction of extra electrolytes via salt simply makes up for the slower rate of osmosis caused by the salt. Again, self defeating.
6. The final thing I want to deal with here is the idea that Salt allows fish to diffuse more oxygen from the water. This is a completely false idea that simply proves that the advocates of this myth simply do not understand fish physiology. There is evidence that things like copper sulfate, salt, and other slime coat boosters can all cause the exact same problem. A good indication of the oxygen content of your water is the ventilation rate of your fish. If a fish is ventilating very quickly then it is attempting to get extra oxygen, and this happens when there is not enough oxygen in the water, the fish is stressed, or it cannot extract enough oxygen from the water. (Edit: author's amendment).
Last edited by fins_n_fur; 10-20-2008 at 09:27 PM.