Here is much of the evidence I've collected to date:
According to Evans, Piermarini, and Choe "The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste"
Physiology Review 2005, the return of blood towards the control pH is primarily due to adjustments of blood bicarbonate concentrations via exchange of acid-base equivalents at the gills. Over 90% of the action occurs at the gills.
Basically, what is boils down to is that the fish exchanges CO2, Na+, and Cl- at the gills until the pH balance between the water and their internals is just the way they want it. Another quote from the above article: "Although variable with the type and extent of the acid-base disturbance, compensatory transport is usually activated within 20-30 min of the disturbance and can reach net-acid or net-base excretion rates of 1,000 micromol per kg per hour."
If I just let the flux rate be 100 micromol per kg per hour for calculation purposes, I think that that means that the fish can change its internal pH around 4 units per hour per kg of the fish or faster down to a pH of 4.0 (after that the time starts increasing exponentially, i.e. 10 hours to get down to 3.0) I actually don't know what the internal pH of a fish is... anyone?. So, smaller fish (smaller kg) can change their pH faster -- makes sense, smaller circulatory system, easy to change concentrations in a smaller volume.
What is really interesting is that the acid-base exchange rate is also dependent upon the salt (Na+ and Cl-) solution, so GH and KH play a much larger role than may be usually suspected. This thread
http
/www.fishforums.net/index.php?showtopic=123070 linked to a site whose author deduced this relationship from experience.
So, it appears if the salts in the water are favorable, most aquarium fish can adapt to a change in pH pretty quickly -- in a matter of minutes really. But, if the changes in salt and total dissolved solids are big, the fish may not be able to use its ability to adjust its pH and that causes shock.
Ion exchange at the gills is important for waste removal also. Fish actually don't excrete ammonia (NH3), they excrete ammonium (NH4+). It is important to know that they excrete the ionic form, because when they want to remove ammonia from their bodies two things occur. 1) Since there is very little or no ammonium in the surround water, the ammonia will diffuse preferentially out of the fish's body. Diffusion occurs down a concentration gradient. That is, it will leave the high concentration, in the fish's body, to go to the low concentration, the surrounding water. This is advantageous to the fish, since the ammonium wants to leave the body naturally, it doesn't have to expend any energy for this to occur. Nature does the work for it. 2) Since it excretes the ionic form of ammonium, NH4+, at the gills the fish has to maintain a charge balance. That is, since it loses a positive ion, it must pick up a positive ion to remain in balance. And the usual positive ion the fish picks up to keep the charge balance? Na+, ionic sodium. Sodium being among the most commonly dissolved minerals in the water. Fish can also use Ca2+ and other positive ions, like potassium, K+. And what is the main measurement we use to know how much positive ions are in the water? The hardness which measures the total amount of minerals in the water.
Maetz and Garcia Romeu
Journal of General Physiology 1964 injected goldfish with NH4+ and reported increased Na+ influx.
The negative ions in the salt, typically Cl-, are important, too, since they are ion exchanged for HCO3- at the gills. HCO3- is the form CO2 takes in the fish's body, and just like mammals, is from the fish's respiration. From Moyle and Cech, Jr.'s
Fishes, An Introduction ot Ichthyology 5th ed. "...ion-exchange mechanisms provide for the maintenence of appropriate internal Na+ and Cl-, elimination of some potentially toxic NH3 (as NH4+), elimination of some metabolic CO2 (as HCO3-), adjustment of internal H+ and OH-, and maintenance of ionic electrical balance."
Note here, that there is two principles at work, diffusion down a gradient and a charge balance; these two principles can work together or can work against each other.
So, how do large changes in hardness affect the fish? Let's do some examples. Consider a fish that goes from high hardness water to low hardness water. The problem here is that low hardness water won't have as many positive ions available for ion exchange at the gills. That means the rate at which ammonium can leave the fish's body is severely hampered, especially compared to the water it was previously in. The fish's body had gotten used to being able to perform a certain rate of ion exchange with its surrounding water, and when it gets placed in water that has much lessor ion exchange capability it take the fish's body a while to re-adjust. And, meanwhile, the ammonium in the fish's body that cannot be exchanged as fast is building up -- poisoning the fish's body, actually. This is why large changes in hardness is tough on fish's body. In this case, the principle of the charge balance harms the fish.
Now, consider the opposite example. The fish goes from low hardness water to high hardness. In this case, the ammonium won't build up because there are ions available for exchange. But, in this case the principle of diffusion down a gradient is what will harm this fish. Because, the fish coming from low mineral content water will have lower mineral content in its system. So, when it is placed into high mineral content water, the minerals in the water are going to want to enter the fish's body. So some extent, that higher concentration of minerals are going to try to flood into the fish's body. Again, there is a period of readjustment that has to occur before the fish's bodies acclimate. This is why large changes in hardness is tough on fish's body.
In both cases, the fish can carry out its normal bodily functions, but they wll have to expend energy to perform their tasks. Like, in the first example, the fish can expend energy to expel the positive ion even though there are no other ions to exchange it with. The energy is expended in order to neutralize the NH4+ to turn it into NH3. In the second example, energy is expended to prevent the ions from flooding into the fish's system. In general, a fish will survive the second example better than the first. But both can be pretty stressful and should be avoided if possible.
Finally, I like to take examples from nature to show that pH is not a large limiting factor in acclimation. Many studies have shown how the pH in lakes and ponds change throughout the day. The sun comes up, heats up the water, the aquatic and nearby terrestrial plants begin their activities, etc. Diana Walstad in here book
Ecology of the Planted Aquarium, has the citations for many of these studies, and graphs of pH in lakes that change well over 1 full pH unit throughout the day. (I am travelling right now and so don't have her book with me to look those studies up.) Another example is quick rains, and.or monsoon season. In general, rain does not have the same pH as the body of water it runs into, especially as it picks up residue and debris from the ground. These quick rains would result in very quick and sudden changes in pH, but I have never heard of massive fish kills after a sudden rain.
Finally finally, I haven't had a chance to look for temperature acclimation. It doesn't seem to be a big issue, since I have found lots of research on thermal regulation of fishes, but nothing about deaths from dramatic changes in temperature. I have found a few really good examples of how fish use temperature differences to their advantage, however. My favorite is from Neverman and Wurtsbaugh,
Oecologia 1994 study of sculpin (
Cottus extensus) living in Bear Lake that would feed from water at the bottom of lake (3 degrees C) but would migrate to upper layers (13-16 degrees C) to acceleration their digestion. Their digestion would improve from 3%/hr to 22%/hr because of the change in temperature. Enzymes and the like become much more active in the warmer water. These fish, on a daily basis, change their ambient water temperature 10 degrees C. If it really took "10 days" for fish to acclimate to new water temps, these wish would never acclimate, ever. I have more leads into the research on temperature changes -- I've seen reports on so-called "heat shock proteins" that form, but I don't know what they are or how their presence effects the fish. I'll try to report what I find when I have more time.
To sum up, I have found lots of evidence that it is the changes in hardness that are the most stressful for fish, not pH and temperature. At this time, I suspect what most people have termed "pH shock" is really hardness shock, since usually water with a different pH has a different hardness as well. My evidence shows that fish can change their internal pH's very quickly if needed, if the minerals are available to perform the ion exchanges. I think that the ion exchanges at the gills really is the key to understanding how fish adapt to changing water conditions.
). Whereas if you've got a delayed shipment of fish from across the country and there's a few DOAs, then dropping the fish in would probably be the best route. I think we can all have an input here.
