Originally Posted by never quit
That's fine, but I thought it wasn't so much the Ph value as it was the CHANGE in the Ph from week to week. If the Ph of a tank falls from 7.6 to 6.4 during the week, then during a weekly water change, it goes back up, wouldn't that cause distress in the fish?
Thanks for your advice
The lowering during the week in this example would be gradual. And I doubt you would raise it back up that much with a water change. The stability of the biological system once the tank is established will tend to maintain fairly constant values for pH, partly determined by the hardness (KH and GH), stocking, live plants, feeding, temperature, etc. Provided the tap water is not too far removed, and here the KH and GH are as important as the pH, the shift in the tank is not going to be significant. As in my tanks, where the pH of 6 in the tank rises to maybe 6.3 with a 50% water change using pH 7 tap water.
The pH stress to fish depends upon the extent of the pH change, both the level and the time during which this occurs. Minor fluctuations are fine, and natural; tropical waters have more flux in temperature and pH than many aquarists realize, the idea that they are static or stable is incorrect.
There is a diurnal pH fluctuation in all planted tanks, which can be as much as .4 or .5, example from 6.2 to 6.6, every 24 hours. The pH is always lowest (most acidic) at the end of the "darkness", and highest at the end of the daylight. This is caused by plant photosynthesis, using CO2 during the daylight which removes carbonic acid from the water thus raising the pH, and during darkness CO2 is released by fish, plants and bacteria adding carbonic acid and thus lowering pH. This occurs in nature too, and fish are fine with it.
During my weekly water change the pH changes (rises) by .3 to .4 within a few minutes, then lowers again over the next several hours.
One author has written that a pH change up to 1 full point, say from 6 to 7, was the maximum fish could handle without difficulty. I wouldn't want to push it that far, but clearly some fluctuation is acceptable.
The pH shock occurs when the change is drastic, such as a major shift in very short time frame. Or a constant significant fluctuation. The afore-mentioned shifts are not within this realm.