OK, well then, here's a little primer on pH, osmotic stres, etc.
Firstly, let me talk about pH. Yes, pH is a logarithmic scale. The p is mathematic shorthand for the negative of the base 10 logarithm (wirtten -log_10) and the H is shorthand for the concentration of H+ ions. So, how does this work? Well, a concentration that has 10^-7 ("ten to the minus seventh power" or 0.000 000 1) moles of H+ ion per liter has a pH of 7.0. The log base 10 of 10^-7 is equal to -7, then the negative of -7 is 7, hence the answer. 1 mole of a substance is 6.022x10^23 molecules of a substance. It is defined like this because in chemistry it is far easier to work in numbers of molecules of a substance for balancing chemical reactions rather than the masses of the reactants.
One really important thing to note here, however, is how small of a number 10^-7 or 0.000 000 1 really is. Now, 10 times that, 10^-6 is 0.000 001. And, while it is a ten-fold difference, both numbers are still really quite small. This is why changes in pH are not as dramatic as they are made out to be. Yes, it is a 10 fold increase to change 1 pH unit, or a 100 fold increase to go 2 pH units, but until you get to pH's of about 2 or 3, we are talking about very small quantities of substances here. And a 10 fold change of a small quantity is almost always still a small quantity. This is why changes of 1 or even 2 units in pH aren't really all that bad for fish, *if the hardness changes aren't too big either* I'll explain this below.
Next, let me talk about osmotic stress. First off, osmotic stress and pH are pretty much unrelated. Osmotic stress is based on the concept of osmotic pressure. The total pressure in a liquid is both a function of the density of the fluid and what has been dissolved in that fluid. The osmotic pressure is the part that is related to what has been dissolved in it. It is useful in determining in what way pure water and/or the minerals dissolved in the water are going to go. As relating to fish, the fish have certain minerals/nutrients they keep in their bodies and of course, the water they live in has certain minerals dissolved in it. Osmotic pressure/stress is NOT a function of the charge of what is dissolved in the fluid, however, so pH being a concentration of a charge, does not have an effect.
Regarding osmotic stress, I want to write first here about how not to use it. Often, people will add salt when their fish are sick, thinking -- because they've seen it written on the Internet and the box of aquarium salt itself says it -- that this will reduce the osmotic stress of the water. Well, firstly, the fish have lived and adapted to the osmotic stress, the difference in osmotic pressures between their bodies and the surrounding water, their whole lives. They do not need to be relieved of it any more than we need to be relieved of the pressure, or the stress, of Earth's atmosphere pressing on us when we are sick. Secondly, the fish actually use that osmotic pressure to perform their regulatory functions.
Here is the promised discussion why the fish use the osmotic pressure and changes in hardness are much more important than changes in pH. We all know fish excrete ammonia as waste. Well, that is not quite 100% true, since fish actually excrete ammonium, NH4+, not ammonia, NH3. Their waste is in the ionic form. Also, fish excrete a large amount of that ammonium via their gills, over 80%. There is some excreted with their urine, but the majority is done via the gills.
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.
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, experimental evidence shows that fish can change their internal pH's very quickly in response to pH changes in the environment. Data indicate that most fish's excrete a net acid flux of 10 to 100 micromol per kilogram per hour under control of steady acid-base conditions. Some back of the envelope calculations indicate that assuming the low number there, 10 micromol per kg per hour, that a small aquarium fish (I did it for a tiger barb) can change its pH over 4 units per hour. A larger fish is going to take a little longer, but the point remains the same, that a fish can change its pH very quickly. Again, this really is best evidenced by nature where the pH in lakes can change 2 units through the course of a day, and again the runoff during hard and fast rains.
This information is taken from the articles: Evans, Piermarini, and Choe, "The Multifunctional Fish Gill" Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste", Physological Reviews Vol 85, 2005 and Claiborne, Edwards, and Morrison-Shetlar "Acid-Base Regulation in Fishes: Cellular and Molecular Mechanism", Journal of Experimental Zoology Vol 293, 2002.
So, there you guys go. Please feel free to ask any questions, I am happy to answer them. But, as a synopsis of the research I presented above, it is the differences in the mineral content -- typically measured as hardness by home aquarists -- that is the real stressors for fish. It is typical that hard water has high pH and soft water has low pH, so I can understand how the connection was made between stress and pH. But, the experiments in the scientific literature just don't back that up. The scientific literature has much evidence that shows that difference in mineral content are much more important to a fish's biological processes, however.
