Hi Hoggie,
Yes I'm pretty sure that the pH in the water has very little to do with fish health. Their external physiology is immune to pH in and of itself. What fish respond to are the elements/agents in the water that have direct effects on their membranes. Salts in the water, and in their tissues, for example have a direct effect on which way water flows. This is why marine fish cannot live in fresh water and why freshwater fish cannot live in salt water. However, the external pH is largely irrelevant.
pH, like nitrate, is blamed for all kinds of evils. That is what The Matrix teaches and that is why the OP is so paranoid about pH. Toxic agents in the water have toxic effects on fish, but toxic agents are not toxic just because of the pH changes that they cause. They are toxic because the chemical interaction of the agent against the fishes tissues have a negative effect by destroying the tissue that it interacts with.
Internally, blood, body fluids, soft tissue, nervous system cells are all affected by pH. This is a different story. the fishes internal physiology has very stringent requirements for proper functions and the internal pH affects the chemical reactions that are part of the life processes. Therefore there are very precise controls of internal body fluid pH primarily through the use of bi-carbonate buffers produced in the bloodstream. But the pH of the external water has nothing to do with what's going on with pH internally.
So this is what I tried to explain earlier. If you have high levels of ammonia in the water column, then the gill tissues, which are very thin and very delicate are immediately exposed to the ammonia. The ammonia directly attacks and destroys gill tissues. If the water pH is high, more damage is done. If the water's pH is low less damage is done. That's because pH has an effect on the behavior of ammonia/ammonium. NH3 (ammonia) is extremely toxic but NH4 (ammonium) is less toxic. At a higher pH NH4 converts to NH3. At a lower pH NH3 converts to NH4. So, very acid waters mitigates, to some extent, the toxicity of agents like ammonia by converting them to less toxic chemical. The thing to understand is though, that it is the chemical agent doing the damage, not the pH. rather than worrying about the pH it is a better policy to eliminate the toxic agent from the water column.
Lets look at the opposite case. CO2 causes the water to be very acidic by lowering the pH of the water. We know that CO2 is a highly toxic substance. But what is the fundamental nature of the toxicity? is it because it causes the pH of the water column to fall? No. It's because CO2 in the water column prevents the fishes body from purging itself of the CO2 generated internally. Normally, CO2 travels from the blood, out to the water from across the gill membrane. High CO2 pressure within the water column prevents this exchange, so the CO2 inside the fishes bloodstream builds to high levels. that then lowers the pH inside the fish's blood. That causes all sorts of problems if their system is unable to control the internal drop in pH. The fish could not care less about the pH drop externally. It is the internal chemical reactions that causes problems and those internal problems are caused by the dissolved CO2 of the water, not the pH of the water.
In CO2 injected tanks, the pH of the water varies drastically rising sharply after lights out and falling drastically during lights on. These pH changes are completely irrelevant, but it is the fishes ability to deal with the internal buildup of CO2 that determines whether they stay healthy, not whether the hobbyist can keep a stable pH.
That's why, within this context, focusing on pH is generally a failed policy, because it ignores the root causes of toxicity. Depending on the toxic agent, the rise and fall of pH is a lottery in terms of how it affects the potency of that particular agent. So buffering the water with a carbonate/bi-carbonate source, just so that it will stabilize the pH, does nothing if the hobbyist does not address the fundamental cause of toxicity. It is much more important to maintain cleanliness of the tank in order to avoid introducing toxic agents in the first place.
Having said all that, the issue of breeding is a different story. Some fish only breed successfully within certain pH and TDS ranges. This is particularly true of dwarf chiclids.
Cheers,
