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I fully agree with the thoughts above. While it seems easy to relate fish death to certain events, it is also very easy to jump to the wrong conclusion. I don't find PH to be a quick killer. I think of water parameters like temperature, PH, KH, and GH, as being somewhat like us being exposed to bad weather. If we spend the night outside in cold weather, we may not like it and we may get sick, etc. but it takes some time to really get there if the conditions are not really extreme. Too much CO2, too little O2, those can kill us or fish pretty quick.
The difficult part may be in reading the signs. Fish have a bad habit of dying when we do something in the tank and it makes us feel we killed them. The truth may be that the fish may have been damaged in a number of ways and just happened to die at that time.

I also like a controller as it does give a far greater control than other methods which plays well with my fish keeping theory. My first big plan to keep fish alive is to not do anything that creates a sudden shift when things are working well. So my method for adjusting CO2 is to make it a very slow and gradual change. I let the controller read the tank for a time ad really get settled, then I begin to creep the controller down just a couple tenths each time and I may take a few weeks to get it down to final point. Where that points winds up is something that my fish let me know. African cichlids are famous for "special" water like hard and alkaline. But if I go slow and easy, I can push my water down from 7.8 PH to about 6.8 without some of them showing stress. But it does vary from one type to another so I can't say exactly how much you should run your tank down.
 

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Part of the reason I question the PH as the cause of death is the way my fish react. I keep and breed African cichlids from Malawi. They are semi-famous as fish who require hard alkaline water and that makes them ideal for my 7.8+PH and 300 PPM GH/KH. But when I find a female holding eggs (mouthbrooders) I catch her and move her from the tank driven down to 6.8 +/- PH into a small tank with no CO2 and 7.8 PH. Moving to that tank there is no obvious stress and she acts normal and spits out the fry with no problem. The only time the females show stress is when they are moved back to the main tank again. The abrupt change from high to low PH seems to make them act a bit slow, with some heavy breathing for a couple hours. No deaths, no hanging near the surface or extreme behavior that calls for action other than watching.
Smaller, more delicate fish of other types may be more prone to death but still it seems extreme, since so many do operate with major PH swings every day/night with no lost fish. If PH change was a major cause, I would expect my new fry to suffer far more but they just pop out and start eating.
Maybe water changes are causing the substrate to release a sudden burst of something that we don't measure? Any chance adding water blows debris around, etc.?
When using a controller, I went with simply leaving all normal and letting the controller turn CO2 on/off to maintain the same PH both day and night. Rather than debate how much swing was okay, I just went with holding all steady as practical to do. I find fish adapt well to steady while they may struggle with frequent or abrupt change.
But then , that is also one of the major problems. All of us will have different situations leading to different results and methods.
 
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