Here's a copy and paste from a previous post:
What is commonly called "pH shock" actually very rarely is caused by pH. 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 it 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."
Just some "back of the envelope" calculations: If I just let the flux rate be 100 micromol per kg per hour, 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.
here's another relevant copy and paste:
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.
lol
