Yes, the pH will go down too when the KH is zero.
It is not something that would occur daily but it may have been happening on and off without your knowledge and existing tank inhabitants could adapt to an extent although it would weaken their immune system long term. New fish added to very acidic water may never adapt.
What is the full list of inhabitants, how old are they now or how long have you had them for?
A few months is a very short term in terms of fish health. The only way you'd know if your water is ok if those fish you got a few months back are still alive a few years later. Fish are tough for the most part but chronic bad conditions will take its toll.
Yeah the only thing I’ve noticed is that bar like one Otto a while back ( which I put down to Pets at Home) all the other loses have been Tetras.
I had 14 Rummy Nose and over the past three months gradually I’m down to 4 now. I had 26 neon Tetras as of last week and this week I’ve lost about 14 of those in tern.
The rest of the habitants are 6 Amano Shrimp, 5 Ottos, 2 Cory Habrosus (grown quit big now) and a small Sting Ray Pleco. Fish are super coloured up and active as they always have been, but tetras from time to time just kinda linger or stay around in one place which I always have found odd. Early on when I got the tank for weeks they would swim back and fourth in big schools.
I don’t feel the Bioload is extreme and I feed little bits at a time. I water change once a week though about 90%.
I wonder if the ferts are affecting the parameters or Kh which could apply some link to that. Could be nothing.
I feel like my plants will start kicking off soon as the water is now so clear from the carbon that it looks invisible lol! I’ll keep it like it for a few days and gradually add ferts again (from the other bottles just Incase.)
Not sure on the co2 if you think acid is a problem as it might be a stressor. The way a Tetra has kinda gone off in sequence makes it almost routine like they are all catching something one by one.
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