How high is the phosphate level? Most people will have 1-2ppm in their tap water and isn't an issue, and even higher values aren't going to cause that much of an issue apart from a little algae growth, and can easily be prevented by adding quick growing plants to the tank or phosphate absorbers to the filter - that is assuming its a freshwater tank.
Also in the main RO water is only necessary for marine setups and not all of them at that
and tbh i'd say 95% of fishkeepers wouldn't have a clue what their phosphate levels were,