... I have cory cats
There are many species of Corydoras...
My pH drops to 5.4 or so during the day and my Cory cats are happy as can be. Don't worry about them, they'll be fine...
RO water has very low mineral content, this relates to a low alkalinity, which makes pH very difficult to stabilize...
At least 2 species of Corydoras thrive in pH levels as low as 4.0 Corydoras melini ? False Bandit Cory ? Seriously Fish Corydoras adolfoi ? Adolfo?s Cory ? Seriously Fish
RO water, rainwater, distilled water, has very low mineral content, this relates to a low pH, it's soft it is acidic and it is very low in TDS (Total Dissolved Solids) - which means it contains a higher ratio of real, pure water. (Less TDS = more water, as water is a liquid not a solid.)
This as opposed to high mineral content, high pH, hard, alkaline, and high in TDS - which means it contains a lower ratio of real, pure water. (More TDS = less water.)
Species which are known to thrive in water of the former description are also much more adapted to fluctuations in pH, as wind, rain, drought, bioload, etc, all cause wider pH fluctuations in "blackwater" regions than in regions of alkaline waterways.
Therefore that it "makes pH very difficult to stabilize" is moot, as acidic water fish have been in it since the dinosaurs.
I notice a troubling trend in modern aquarium keepers, where the measure of welfare seems to be steeped solely in terms of survival: if the fishes live, things are good, if the fishes die, things are bad. It is an inappropriate position to take. [Nathan Hill in PFK]