Hi all,
Darrel, is that why pH swings CAN be dangerous with very soft water ? Or how does soft water come into effect in such situations ?
Acidosis
Acidosis and "Old tank syndrome" can be a real problem, but the low pH and fish death are actually both symptoms of what has gone wrong, rather than the low pH causing fish death.
pH, the measurement
The real problem is that pH is such a strange measurement. To get your head around it, you need to think of your water in terms of the amount of H+ ions, the ratio of H+ ion donors (acids) and acceptors (alkali or bases), and the reserve of bases (or dKH). The really important think is that pH isn't a measure of amounts, it is a measure of ratio.
pH doesn't tell us anything about amounts, just ratios.
If I use the scales analogy, in soft water at pH7 you have a sugar cube in either pan (the cubes represent H+ ion donors = "acids" in the acidic pan, and H+ ion acceptors ="bases" in the alkaline pan), the ratio gives you pH, and it is 1:1, 10-7 each of H+ and OH- (O-H) ions
But we will also have pH7 if both pans have a kilo bag of sugar, the ratio is still 1:1, and the pH7. In the soft water case adding a sugar cube of H+ ion donors will reduce the pH down to somewhere near pH4, in the bag of sugar case it will have virtually no effect on pH, because we are talking ratios not amounts. My tap water, which comes out of the tap at about 650 microS, pH7.8 and is saturated with calcium carbonate. To reduce the pH I'd need to add enough acid to neutralise a huge amount of carbonate buffering, in this case even small movements in the pH reflect large changes in water chemistry.
Carbonate buffering
Where a low reserve of carbonates can have an effect is in biological filtration, where the conversion of NH3 - NO2 - NO3, uses both a lot of oxygen and a lot of carbonates. If you have a very low reserve of carbonates (dKH), pH values will be unstable. Another problem you may have is that heavy metals like copper (Cu++) will be potentially in solution at acid pH levels, and again these may effect both fish and biological filtration.
Old Tank Syndrome
If you don't change very much water volume, the natural processes in the tank over time will reduce the pH and the reserve of carbonate buffering. This is "old tank syndrome", which was a common problem in the 1970's, where most people didn't change their water, and old tank water was supposedly full of magical properties, and fresh water was thought to be actively dangerous, I started fish keeping then and killed all my fish fairly quickly.
There is a good explanation here by the ever erudite "Skeptical Aquarist": <
http://www.skepticalaquarist.com/bioacidification>
This is the stable, highly carbonate buffered, hard water scenario, where even though the "lowers the pH" side will grow larger over time, it will always be a tiny fraction of the "raises the pH" buffering.
and this is what happens to the pH when the dKH (conjugated base reserve)is exhausted.
cheers Darrel