There has been a lot of discussion lately about salt and nitrite. The general invalid assertion I am discussing is that with
salt, fish can be kept alive indefinitely. I will attempt to add a little perspective to the subject.
First off, the koi has to pick up lost salts from its environment, as the koi has to maintain a balance of 0.9 % salt solution
in its body fluids (this is the equivalent of 1.5 ounces salt per imp gallon). The koi will be continually losing salt by
diffusion across the gill lamellae.
Diffusion occurs with a solute (in this case salt) migrating from a higher to a lower concentration in an attempt to equalize
the concentration. Once equalization is achieved, diffusion ceases. In the case of the koi, diffusion ceases when the
0.9% internal concentration equals the outside ambient salt concentration. As the external salt concentration should
never be more than 0.3% on a permanent basis, and even this high figure of 0.3% is contentious, It is easy to see that
the koi is continually losing essential body salt. So, the koi must pick up salt/chloride, which it gets from background
salt levels in the pond water. Again, diffusion occurs across the gill lamellae via special cells, called columnar cells.
These cells actively seek out chloride ions; they also work against the concentration gradient, depositing the chloride ions
in the blood stream (i.e., from a low concentration to higher one).
The problem is that these cells although actively seek chloride ionís, they are not that selective in what they pick up;
hence, they will pick up NO2 (nitrite) ions. So, what do we do if we have nitrite ions present? We simply out-number the
nitrite ions with chloride ions. The common statement about chloride ions competing with nitrite ions for entry to the gill
whilst gets the point across, is perhaps a poor choice of words in this instance, as it implies that there is some direct
conflict/ battle between the two ions taking place, the answer is far from being that complex,
If we achieve a ten to one salt to nitrite ratio in the pond water, the gill still has a ten to one chance of picking up a nitrite ion, which will occur if one comes into range of these special columnar cells in the gill.
Imagine you have a bag with 100 balls and 50% represent nitrite ions and the other 50% chloride ions, you have a
50/50 chance of picking up either a nitrite or chloride ion. Imagine the same mixture, but now the bag has 99 chloride balls
and only one nitrite ball, there is a one hundred to one probability of picking up a nitrite ion. Salt does nothing to alter
the toxicity of nitrite, we as koi keepers, just lessen the impact of nitrite by out-numbering the NO2 ions with the addition
of salt. The koi will still diffuse NO2 ions as they come into range of the gill columnar cells.
Nitrite Toxicity (how is nitrite toxic?)
If the nitrite ions enter the blood stream, it oxidizes the hemoglobin to form methemoglobin - methemoglobin is absolutely
useless as an oxygen carrier. A hemoglobin molecule contains four sub-groups (haem- groups) also each haem- contains
ferrous iron, which gives the blood its red color and is also responsible for carrying oxygen. Each hemoglobin molecule
can carry a maximum of four oxygen molecules, or up to 70% of the amount of O2 held in simple solution in its
environmental pond water at any given O2 saturation. When a hemoglobin molecule is exposed to nitrite, its ferrous
iron is oxidized by the Nitrite to form ferric iron. This not only changes the bloods color from red to brown
(brown blood disease), but it also alters the haemoglobinís affinity for attracting oxygen and further is incapable
of oxygen transport.
The hemoglobin molecule is now known as Methaemoglobin, and this is the prime explanation for the toxicity of
Methaemoglobin. Once this happens, the hemoglobin molecule is finished, no amount of salt will repair this damage until
it is de-selected by the spleen and recycled to form new haemoglobin, but replacement by the spleen and kidney has
its own problems, because oxygen transport is now deficient Ė systemís which drive the
spleen and kidney to replace haemoglobin will detect this deficiency in Haemoglobin/oxygen, and will go into overdrive producing extra hemoglobin
to pull back the shortfall. The spleen/kidney cannot indefinitely produce excess hemoglobin if further nitrite poisoning
As haemoglobin production resources are used up by continual over production of haemoglobin, the fish will soon
die of anemia, indicated by pale pink gills.
Also, note that chloramine does the exact same thing to hemoglobin as nitrite (oxidizes it to form methemoglobin),
which is a good a reason for not running low-level NO2 readings (No2 levels should be zero (un-measurable)).
Another Phenomena that will alter the bloods affinity for attracting O2 and the carrying mechanism of o2, are a very low
or acidic pond water ph.
As an acidic environmental change in the blood chemistry is essentially the haemoglobinís cue to off load newly acquired
o2 into the body tissue and to acquire spent O2 in the form of CO2 carbon dioxide and H2CO3 carbonic acid for exchange
at the gill.
The spent oxygen in the tissue forms carbon dioxide, which in turn dissolves into fluid in the tissue as carbonic acid,
thus lowering the body fluid Ph at this point, which, is the cue for the haemoglobin to release to O2 and pick up
the CO2. On arrival at the gill if the haemoglobin is not met by a slight rise in ph the haemoglobin will want to hang on
to the CO2 and not exchange it for O2 to continue the cycle.
Why do koi die continually from nitrite poisoning?
The above explanation is why a higher chloride to nitrite ratio or at least as high as is safe and practical, demonstrates
why It is believed a 10 to 1 ratio is at best better than nothing but less than adequate, as this concentration will be
found in most domestic water supplies,
And why continual low level nitrite will have a detrimental effect on the long term health of the koi A 0.3% saline solution
- is much easier on the fish short term. Also, itís very important to get to the root of the problem to eliminate nitrite
(or chloramine) in the pond. Otherwise, sooner or later you will pay the price with dead fish.