DrVader - I agree that cell bound organelles require pH gradients; what I'm not at all convinced of, either theoretically or experientially, is that the pH of the water the fish swim in has a direct and immediate effect on the pH inside their cells.
Big sudden changes - say more than two or three points - may well be more than a fish can cope with in terms of maintaining the right alkaline blood pH, but how do we define "sudden"? The way some people talk, you'd not think fish would survive in many ponds where pH fluctuates as CO2 is used up by the flora. And when I hear of people getting worried about a pH that's below 8, I feel pretty sure there's a great deal of mythology around the subject. I've heard people claim pH shouldn't change by more than a tenth of a point a day - where does that come from? And who's got a test kit that's that precise, even allowing for the fact that the colours the test solution goes all look more like each other than any of the ones on the card (IME; naturally YMMV)?
The blood pH is very important; so much so that any fish species wanting to survive has to be very good at maintaining it whatever the outside pH is. My reading has found that fish can adapt to a full point pH change within around an hour (depending of course on the size of the change). It's difficult to find many papers on the topic because researchers are mostly interested in what happens if you stick a guppy in pH 3.5 (it dies, not from the pH
per se, but from inhibited uptake of Na+; it just can't stick H+ into the water against the concentration gradient in order to take Na+ in).
Sometimes I think hobby kits should only measure pH to the nearest half a point; frankly, unless you're calculating ammonia toxicity (and a better approach is Water Change Until You Can't Measure It Any More anyway), then knowing the pH better than "around 6.5" or "around 8" is perfectly adequate. Given that our domestic aquaria tend to range between 6.5 and 8.5 and only go outside those ranges in the most unusual of circumstances, I'd say that for nearly all the fish we keep we can pretty much stop worrying about pH, especially since if you give amazonian fish the low TDS they want, you'll get the low pH anyway, and similarly for Rift Lake fish - bang up the hardness by any conventional means and the pH will rise anyway - that the precise ranges of pH the fish can cope with and the degree of adaptability to pH they possess is pretty much academic. There are so many other factors we can't know about as well - if the pH is the same, and the TDS the same, but the [Na+] decreases by a factor of 10, how long does the fish take to adapt to that? Does it die before it can adapt? Who knows?
Meanwhile AP make a fortune out of their "Proper pH" products by propagating the pH myth
