I have never seen a reaction from a killifish due to a sudden pH swing. I routinely have subjected my fish to sudden changes of 2 or more points, and Col Scheel, in his Atlas, says 3 points show no adverse effects.
Can you feel the pH when you go swimming or take a shower? Well, neither can the fish.
What can the fish feel?
If the pH goes suddenly upward, and there is a lot of ammonium present, it can turn to ammonia and burn the hell out of gills and skin. The percent as ammonia can go from 0.5% at a pH of 7 to 36% at a pH of 9. Babies are stunted at levels as low as 5-8 ppb! At 36% nearly all fish die if there is any measurable ammonium at all.
If the pH is tied to dissolved solids, a large pH difference may mean a large difference in total dissolved solids (tds), and sudden change in that can cause gill cells to burst as the cell-wall osmosis regulatory system is too slow to adjust and keep low tds water from diluting the body fluids. This is called osmotic shock, and is more common when going from harder (high tds or salinity) water to softer water. I try to keep sudden changes below a factor of 2, which most fish seem to tolerate OK.
Low pH can interfere with nitrite conversion to nitrates and lead to "Brown Blood" disease, but it is rarely a fast transition problem. It is more a "pH Crash" situation when water changes are missed and the water quality degrades. Low pH can lead to heavy-metal poisoning, too, as lead and copper are more soluble then. Don't leave lead anchors on your plants!
With pH dependent chemistry like this, it is easy to see where the pH mythology got started. We have very cheap GH kits, tds meters and such, available now, so there is no need for us to be controlling and/or measuring the wrong thing.
IMHO, pH shock, per se, is pure myth.
Wright
01 760 872-3995
805 Valley West Circle
Bishop, CA 93514 USA
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