I always found rainwater to be somewhat unstable, especially with the pH
(usually dropping suddenly). Most problems I had with rainbows usually
related to low pH. It would probably be alright if you added some kH
additive to boost the alkalinity.
I know Rainbowfishes are found occasionally in low pH water, but it doesn't
seem to suit them in captivity?
As for temperature - the survival rates for Rainbowfishes in their natural
habitats declines sharply when the water temperature is high and will often
die at temperatures above 36°C. Whether this relates to the temperature or
lack of oxygen, I don't know?
Also, in Brisbane the Council seem to boost the free chlorine levels in hot
dry weather when demand is highest and you may get water from a different
source than you normally do as well.
Adrian.
Adrian R. Tappin
Home of the Rainbowfish
http://www.powerup.com.au/~tappin/