RO/DI takes everything, including the things your fish need to properly osmoregulate.
For salt water tanks, it is good because the sea water mix replaces all the things the animals need, but unless you add back those trace minerals the fresh water fish need, you could be causing another form of stress.
Many times the water provider has to dose chemicals according to the quality of the source water. If the source is contaminated with coliforms from runoff, it must be treated accordingly to prevent human disease. This often comes into conflict with what fish keepers need.
You should be able to find out what your provider doses by googling the name of your city or county, followed by "water quality report". All water providers are required by the EPA to submit a yearly report, which its customers can view.
I have googled many MFK member reports, but you do not have a city listed below your avatar, or I would have looked it up.
Chlorine/chloramine levels may change drastically with the quality of incoming water, but must remain within national MCL limits and guidelines
I found when I used pure RO/DI in the lab, even bacteria would die, they would lyse because the lack of trace elements would cause their cell walls to rupture.
Especially with cichlids like Cubans and Mayans, I doubt RO/DI would be a good choice, unless you mix it with tap water (that you should be de-chlorinating). These cichlids come from highly alkaline waters, that are often called "liquid rock".
