Rainwater?

[BillD]

All rain water is acidic, usually with a pH of 5 or less.

This is contrary to my experience. I haven't checked for years but when I did, the pH was 7 or a bit above. At the time, I was hoping it would be more acidic.
I believe that TDS is far more important than pH in terms of keeping water uniform. If you look at what happens in Costa Rica, where water is not typically soft as it is in many parts of south America, they can have water levels in rivers rise by 30 feet or more in one day. This happens numerous (as many as a dozen ) times per year unlike the once a year rainy season in SA. This has to have a sudden drastic effect on pH and TDS. The fish seem to survive this without any issues.

 

[Caliban07]

What would happen to the TDS in this instance?

 

[Delapool]

Would you put it in your fish tanks?

From a large rain tank in the country I would, a small tank in the town I wouldn't (I'd go with tap water and/or RO stabilised).

Summer here has no rain so it would tend to run dry and I'd be concerned on stability of the water specs and what it is picking up off the roof. It's just little things like the water would be freezing in winter and getting warm in summer.

Guess I'm spoilt really