Quick take
For many freshwater tanks, conditioned tap water is appropriate. RO or RO/DI becomes useful when the aquarist needs precise control over hardness and dissolved solids. Always rebuild minerals to the target chemistry for the livestock being kept.
Tap water can be perfectly usable
Municipal water can work well when disinfectants are properly treated and the natural hardness suits the species.
When RO helps
RO gives the aquarist a low-mineral starting point that can be blended or remineralized to a controlled target.
RO/DI for very low dissolved solids
Marine and specialty setups often use deionization after RO for even lower residual ions.
Avoid generic livestock advice
Aquarium chemistry targets vary widely by species, so filtration should support a known target rather than chase “pure water” as an end in itself.