The Salt of the Earth, the Salt from the Sea . . .
We say (or at least my grandmother said) that someone is “worth their salt” or perhaps “not worth their salt”, meaning that the person under discussion justifies their existence (or not) by their actions. Can we apply this same sort of value system to the mineral in question itself? Is salt (sodium chloride, NaCl) worth its salt in the freshwater aquarium? Disclaimers are needed first: This is my personal opinion, not the official doctrine of this site, but just me, the cranky old fogy who has rather strong opinions on a number of fish-keeping topics.
Salt has historically been a commodity of no small economic importance. It is part of our blood and, as a necessary electrolyte, a requirement for our life. We lose salt in sweat, and in tears, as well as the more conventional fluid disposal process. Salt is and has been throughout history used to preserve food as well as to enhance flavors. It is also used in modern American suburbs in water softeners- to which we will return later. It has for some – to me strange – reason become popular as a routine addition to freshwater (FW) fish tanks. That is the primary subject of this article.
Sodium chloride is the familiar white crystalline material in just about every kitchen. Table salt as purchased at the local supermarket in this country also contains iodide and anti-caking compounds.
Iodine is a halogen, and is required for vertebrates in its ionic form. Iodine is the element; iodide is the ionic form. Do not confuse either of those with “tincture of iodine” which is a topical antiseptic and quite toxic. Iodide is necessary for our metabolism as an essential part of thyroid hormone, which is our metabolic pacemaker. Soils in wide areas of this country are deficient in iodine, and this lack can result in goiter (hypertrophy of the thyroid gland, effectively from insufficient iodide intake). Thus the practice arose of adding iodide to salt intended for human consumption. This was the safest (the levels of iodide are minute) and surest way of protecting the population from this deficiency as salt is ubiquitous (all but universal) in food processing and preparation. The levels of iodide added to table salt are so small that any water-living vertebrate or invertebrate would be pickled in brine well before toxic concentrations of iodide could be reached, so that particular urban myth is without foundation. In fact, a number of our tank inhabitants need iodide- most crustaceans have a significant demand for the material, and a number of fish can develop goiter in captivity from the lack of iodine- African Rift Lake fish seem especially prone to this. The often-discussed toxicity of iodide could be considered urban myth #1.
Salt, sodium chloride, is hydroscopic- exposed to air of more than Sahara-at-midday humidity, it will pick up moisture from the air on the surfaces of the individual crystals, which melt at the surface and cement themselves together- in short, they clump. The salt shaker does not work with clumpy salt, so additives are used to block the clumping. Arrowroot is common for this purpose, but others are possible. Again, as with iodine, quantities are small (but larger than with iodide), and are food-safe and fish-safe
Kosher salt is commonly suggested as an alternative to table salt, as it does not have iodide added. This of course is a response to salt myth #1. Others gasp in horror at this suggestion, as kosher salt may have yellow prussiate of soda (the sodium salt of prussic acid, a ferro- or ferricyanide) as its anti-caking agent. Horrors! That is a cyanide compound! You are sending your fish to the gas chamber and it will kill them instantly! Horsefeathers. Once again, the quantity is tiny, food-safe, and the fish would be pickled in brine long before potentially toxic levels could be reached. Salt urban myth #2 down.
Now for the biggie, urban myth #3 (with subsections a, b, c, d, and e – for my convenience): “You should always add x amount of salt to your tank water.” The quantity “x” can vary with the poster and the local mythology, but generally is on the order of one tablespoon per five gallons. Note: this advice may be given with no questions asked about water hardness, nature of the fish kept, presence or absence of live plants, or actual water purpose or aim of the tank. This is presented as one of the great revealed truths about captive fish husbandry, universal, rarely if ever qualified or restricted, and eminently suitable for carving in stone over the door of your fish room. It slices, it dices, it chops, and your fish will live happily forever and ever. If you detect a trace of sarcasm, you begin to understand my attitude toward such edicts.
A small but significant digression about water in nature, in the USA, and in our tanks before dissection of the big myth: We commonly measure water hardness of our tap water by general hardness (GH) and by carbonate hardness/alkalinity (KH).
General hardness can be expressed as ppm or degrees. One degree of general hardness in the USA is the equivalent of 17.86 mg CaCO3 (calcium carbonate) per liter (1 mg/l = 1 ppm for our purposes). GH is measuring the concentration of calcium and magnesium ions (Ca++, Mg++) in the sample. The term arose historically because water with higher levels of Ca and Mg is more difficult to use for washing clothes – it is harder to produce lather from soap (or from detergent), hence “hard” water. Please note that sodium (Na+) is NOT measured in this test. Common ion exchange resins, both those used as pillows in tank filters and in bulk in household water “softeners” commonly exchange Na+ for Ca++ and Mg++. Further note that by the nature of the resin chemistry, the charges must balance. Two Na+ ions must be added to the water for each Ca++ (or Mg++) removed. The output water will be softer by the laundry definition of soft versus hard, but the water now contains more ions than it did before the softener. The total dissolved solids (TDS) in the water are higher than that with which we started. But the GH test shows lower readings. The importance of this for fish-keepers is that those fish for which we want “softer” water really do not want “soft water” (they don’t do laundry), they want water with low TDS which would include low GH as we measure it, but also low Na+ and Cl-. The Amazonian fish and some of the SE Asians and riverine Africans come from waters of low TDS- yes, the water would read low in GH, but a lab reading of sodium would be low as well, where ion exchange resins lower GH by markedly increasing sodium. If you want lower TDS water, you must go with peat extraction, de-ionization (DI), or reverse osmosis (RO). Otherwise you are kidding yourself that you are bringing the water closer to the fish’s natural or native water parameters. That is urban myth 3a.
Carbonate hardness or KH is a measure of the water’s natural buffering capacity, commonly and confusingly referred to as carbonate hardness (potentially confusing with the other hardness), or as alkalinity (although the connection to pH is a bit backhanded). In most natural waters, the primary buffering system is carbonate-bicarbonate (CO3–, HCO3-). Adding most LFS buffer products is an exercise in futility if the aim is to move the pH to some other level than that of your tap water after aeration, unless your water is KH 3 degrees or below (not usual but certainly possible in the USA). This practice is most likely to stress your fish from the constantly changing pH resulting from such additions. Those additions are of course also increasing the TDS of the tank water. As many of these buffers are phosphate-based, it is also common for such use to contribute to algae outbreaks due to excessive build-up of phosphate. The carbonates may be removed by the ion exchange resins mentioned above (traded for Cl-, chloride). Chlorides have no influence as buffers. Carbonates are also removed or reduced by RO or DI. Excessively low KH (<3-4) is a hazard, as the tank has no other process for neutralization of the acids resulting from nitrification of ammonia (potential for pH crash), or from carbonate utilization by plants seeking a carbon source (potential for pH skyrocket from biogenic decalcification). Addition of sodium chloride to low-KH water for the benefit of African Rift Lake Cichlids or brackish water fish increases the osmolarity (another measure of the total ions present in the water), which to some degree is beneficial. But it does nothing, repeat nothing, to buffer the water to the alkalinity preferred by these fish. That is urban myth #3b.
Urban myth 3c is the idea that salt is tonic for fish when it is kept in the tanks at moderate levels (~ 1 tablespoon per 5 gallons is one common suggestion). If your fish are the most commonly kept community fish of Tetras, corys, angels and such, or even the Rasboras or most Anabantids, these are fish largely from soft acid low-TDS water. It has yet to be explained to me how the average USA tap water, which is at least moderately hard to hard, and alkaline, is going to be improved by increasing those very things in which our water differs most from the natural waters of these fish. Certainly many to most of these fish adapt well to our local water conditions, and I am one of the strongest supporters and advocates of adapting non-breeder fish to local conditions. This is far better for both fish and keeper than constantly battling sew-sawing water parameters. But the idea that the water is improved for these fish by adding still more TDS goes right by me – I just cannot grasp any meaning to this.
Urban myth 3d is related to 3c, dealing with the prophylactic use of salt to avoid common infectious diseases of fish by keeping the level of salt detailed above in the tank. I am a strong believer in and advocate of the use of salt, usually accompanied by increased temperatures, in the treatment of the commonest ectoparasite we see, Ichthyophthirius multifiliis, or Ich, a ciliated protozoan. Only the free-swimming form of this parasite is treatable. Neither the fish-embedded growth phase, nor the encysted multiplication stage that rests on or in the substrate is readily attacked. However, I am not quite convinced of the efficacy of the prescribed dosage (I use at least 1 teaspoon per gallon actual water volume). Also I see no reason whatsoever to “treat” a non-infected tank on a chronic basis. If “new” (to you, your water conditions, and your care) fish are isolated in a quarantine tank for four weeks minimum, they can be observed to be healthy and parasite-free before addition to a community situation. What is the need of treatment in the display tank? Should, by whatever happenstance, the display tank show Ich, certainly treat it. Afterward the added salt can be cleared by regular water changes. Why maintain a treatment regimen in a recovered and healthy tank?
The final urban legend is 3e: The addition of NaCl at the same 1tablespoon per 5 gallons as beneficial for livebearers. Here the pendulum swings closest to agreement with the myth. These fish as a group are native to estuarine environments where the waters are likely to be hard and alkaline at least, and may be brackish. Sodium chloride is matching their home, right? Well, maybe. If your water is moderately hard to hard (GH 8-12 or more, KH the same range), then some NaCl addition may be all that is needed, if anything is needed at all. If your water is soft and acid, you need buffering and increased TDS as much or than the addition of specific minerals to the water, and NaCl alone will not be sufficient. You need to add crushed coral or aragonite, perhaps plus salt, or just the use of marine mix rather than table salt alone. The marine mix contains the whole range of cations and anions (positively and negatively charged ions, such as Na+, Ca ++; Cl-, CO3–) found in the sea, and will be more than adequately buffered. The use of “sea salt” (a table grade of culinary salt from evaporation ponds of seawater) is not an adequate replacement. Brine in these evaporation ponds undergoes a large number of interesting reactions during the concentration process, so the final product does not match the composition or functional ability to support life that was present in the original seawater or in water made up from marine mix. It is fine for table use, but inappropriate for the tank due to the significant differences from seawater.
So, is salt worth its salt? Definitely. It may be a part of the treatment of choice in certain conditions during an acute infection. It may even be part of the proper regimen for long-term maintenance of certain types of fish. Should it always be in your FW tank regardless of your fish or your water? In my humble opinion and in my experience, absolutely not.
This article was originally written for another site. It has been edited for use here. March, 2010
Robert T. Ricketts, a.k.a. RTR