Cheers
@Byron
Neon tetras
Zebra danios
Corydoras
Dwarf gourami
Amano shrimp
I read somewhere, can’t remember where, that a low KH destabilises the PH.
First on the fish...these will be fine with what you have. And believe me, once you start messing with parameters (attempting to adjust pH or KH or GH) you are getting into a very risky business unless it is done carefully and correctly. When there is no need, this is only adding a complication. I am thinking of the fish here, I've no experience with shrimp, and some need higher GH/pH but I believe there are some that don't.
To the last point in your post...there is a very close connection between GH, KH and pH. Generally, the GH and KH tend to be close, i.e., if the GH is say moderately hard, the KH will be relative, and the pH will tend to be on the basic side. The KH serves to buffer the pH, preventing it from fluctuating. This is why attempts to lower the pH by chemicals when the GH and KH are high are so dangerous; the immediate lowering from the addition of the "acid" is temporary because within 24 hours or less the buffering capability will return the pH. This is very dangerous to fish; a stable pH is always safer. However, this can get misunderstood by many.
There is a natural diurnal pH shift in most (maybe all) natural habitat waters of a few decimal points. This occurs in an aquarium, and is relative to the dissolved CO2. In tanks with a good plant load, this is quite obvious; the pH in the very early morning after the dark night will be at its lowest, while the pH at the end of the "daylight" period will be at its highest; in my own tanks this can vary from 6.2 to 6.6 every 24 hours. Fish have no issue with this, it is natural, and the change is over 12 hours each way so it is gradual.
I let the pH do what it wants. I have zero GH and zero KH in my tap water, and in my tanks. The pH will obviously lower, but if I leave these alone, and just do regular (weekly) partial water changes, and do not overstock or overfeed, the system develops its own somewhat unique biological stability, and it does not vary so long as I leave it alone. I keep fish native to soft and very soft waters. So I am in effect providing almost identical parameter conditions to their habitats. The fish have evolved over thousands of years to function in such an environment, so there is no reason to think they will not be at their best with this approach.
If I were to keep fish from harder water, it would be very different. I would need to increase the GH and KH, and the pH would naturally increase in proportion. I have gone down this road in the past.
