Good info RD. I've just learned the hard way - ordering fish from vendors keeping them in pH8 water, only to have them crash and die within a day in my pH5 water. Cloudy eyes and "burnt" fins....and it's definitely not ammonia. That's 1000x more free H+.
It's possible that SOMETIMES pH difference is just an indirect measurement of TDS differences. But think about pH crashes in existing tanks, which happens often on my only bare bottom tank. Fish become very uncomfortable and, in a couple of cases, die within hours or a day. I don't think it's TDS change as nothing is added to or taken out of the tank, and TDS is close to zero to begin with. Only thing I can think of is pH dropping from 5-5.5 to 2-3. My water is too soft and there's virtually nothing that buffers any pH change. When they die, they have the same "cloudy eye" and burnt fins.
A more extreme example is what a friend of mine recently went through. He's an experienced discus breeder and bought one of those giant totes to store his RO water. He was told that the tote was food safe and saw nothing in it, so he went ahead and used it. Turned out there's trace amount of HCl in it that turned the water to pH2, and killed some of his breeders in a day. I wouldn't think there's much TDS change involved.
On the other hand, I sometimes have to salt my fish. When I do, I use 1lb 10oz per 100g. That's a huge amount of salt when you think about TDS changes, perhaps hundreds of fold increase in a few minutes. Never seen any fish looking uncomfortable short term or long term. Any fish, even the wild discus.
So these are the examples of pH changes independent of TDS changes that kill fish, and drastic TDS changes with no pH change that don't have any effect. Perhaps pH changes are not the only factor here, but I hardly think it's a myth.
It's possible that SOMETIMES pH difference is just an indirect measurement of TDS differences. But think about pH crashes in existing tanks, which happens often on my only bare bottom tank. Fish become very uncomfortable and, in a couple of cases, die within hours or a day. I don't think it's TDS change as nothing is added to or taken out of the tank, and TDS is close to zero to begin with. Only thing I can think of is pH dropping from 5-5.5 to 2-3. My water is too soft and there's virtually nothing that buffers any pH change. When they die, they have the same "cloudy eye" and burnt fins.
A more extreme example is what a friend of mine recently went through. He's an experienced discus breeder and bought one of those giant totes to store his RO water. He was told that the tote was food safe and saw nothing in it, so he went ahead and used it. Turned out there's trace amount of HCl in it that turned the water to pH2, and killed some of his breeders in a day. I wouldn't think there's much TDS change involved.
On the other hand, I sometimes have to salt my fish. When I do, I use 1lb 10oz per 100g. That's a huge amount of salt when you think about TDS changes, perhaps hundreds of fold increase in a few minutes. Never seen any fish looking uncomfortable short term or long term. Any fish, even the wild discus.
So these are the examples of pH changes independent of TDS changes that kill fish, and drastic TDS changes with no pH change that don't have any effect. Perhaps pH changes are not the only factor here, but I hardly think it's a myth.