Are you adding supplements to the RO
water? Is there are reason you're using RO
water in a freshwater tank instead of tap water?
The biggest problem with using distilled / RO
water is that it is completely void of the minerals and trace elements that are used by many freshwater fish to survive.
Saltwater aquariums use RO
water to prevent unnecessary minerals like copper and such from getting into their systems - but they replace the "good" minerals and trace elements either through directly dosing them - or buying saltwater salt mixes that include them.
If you are just using RO
water for your tank, and not supplementing it with the beneficial minerals and elements - your Mollies could be suffering from deficiencies.
Unless there's something seriously wrong with your tapwater that makes it unsafe for fish - I'd save the RO
unit for a saltwater tank and just go back to water changes with tap water for your freshwater tank.