Yeah, since you've been raising the alkalinity and then adding your acidic (low KH) source water, then this likely stressed the fish. So, as I mentioned in
post#6, there is quite a bit of changing going on here in addition to your diurnal pH changes. A KH reading of 1 dKH (or 17.9) can mean two things: 1) you have a 17.9 ppm alkalinity; 2) you have no registrable alkalinity. This is where the digital alkalinity instruments come into play, which is not necessary for us in our context unless it is just something you want to do. Most likely, if the test kit registers 1 dKH, then there is no registrable alkalinity.
If this well water is your only source of water, then I would only do big water changes once or twice a year. Using this water for top offs, due to evaporation, will alone eat a noticeable amount of alkalinity if your "top off" volume is quite high.
An alkalinity of 5dKH (that is 89.5 ppm) is not high enough to stop the diurnal pH changes. A heavy residue of algae or heavy residue of plant decomposition or heavy rains or significant water "top offs" from the acid well water or increase in fish density will allow these changes. However, with a 5dKH, excellent water circulation and excellent aeration will significantly help since it is the carbon dioxide fluctuations that is the primary culprit of these pH changes and the water circulates/aeration helps to rapidly release the carbon dioxide before it has a chance to significantly impact the water pH. The amount of water circulation and aeration is entirely dependent on the pond and is different for everyone.
If you want to permanently stop the diurnal pH changes, then an easy fix is to increase the alkalinity to 7dKH (125ppm) or higher. At around 7dKH, the pH will be around 8.3~8.5 (+/- 0.2 variance or so) and any alkalinity levels higher than this will remain around 8.5 (+/- 0.2 variance or so). The only time the pH will will go higher than this is due to a lower free calcium concentration (relative to your carbonate presence) or due to water additives. Also, good to use a presence of a slow release calcium carbonate product to you help maintain, which is sometimes not always enough so you might still need to use baking soda. My previous posts in the threads of this Chemistry sub-forum talks further about all of this in detail and how to approach it.
Once your alkalinity is 7dKH, then the general rule of "1 dKH every 24 hours" no longer applies. So, if your alkalinity is at least 7dKH and you know a very heavy rain storm is coming or before you leave for a long vacation, then put in enough baking soda for around 3dKH or more.
My source water alkalinity hovers around 12~15dKH (214~267 ppm) and the fish show no issues with this high alkalinity.
Only time to be concerned is dependent on fish species and if they can tolerate will enough a pH of around 8.5.
If you want to obtain a more "natural" pH and alkalinity level, then significantly increase the use of calcium carbonate. Depending on the calcium carbonate product used (as talked about in
post#6 in thread Raisng KH with Coral or Oyster shells) and how it is installed, then the pH might stay around 7.4~7.8 with negligible diurnal pH changes (assuming you have excellently suffucient water circulation and aeration).
Just appropriately raise your alkalinity, only do those big water changes quite slowly (slower than over a period of 8 hours) once or twice a year while maintaining your alkalinity, and everything will be quite fine. Your fish will recuperate if they were not already sick before all this.
There is also other pathogens and contaminants being removed with the water changes as well. There is no natural lake or reservoir that does not have an occurrence of water changes. Baking soda does contain a small amount of sodium so, over a period of a year, your salinity will increase, which is not a good thing. So, once or twice a year I would still recommend a slow big water change.
