0.5ppm on your device can refer to 0.72μS/cm or 1μS/cm electric conductivity depending on which ppm scale you have selected. For the 500 (0.5) scale it would be 1μS/cm and for the 700 (0.7) scale it would be 0.72μS/cm. As long as you stay below 1μS/cm even a sensitive system should not have any issues.
Using 1.5μS/cm or 2μS/cm RO/DI for too long as top off water causes my (arguably very SiO2 sensitive) tank to develop brown stains (Diatoms as verified through a microscope). It's a useful indicator to replace the DI resin cartridges and can be used to reduce nutrients, but if you are aiming for stability, a lower conductivity is better.
Note that there are different forms of ion exchange resins. Some will exchange ions with H and OH ions causing a significant drop in conductivity, while others will exchange ions with Na and Cl ions, which doesn't reduce conductivity as much, but still gets the job done to remove unwanted substances. So if you are using Na / Cl form resin, you might have a higher threshold than 1μS/cm before issues arise and of course it depends on what's in your tap water and your tank. There are quite some people which use tap water and have no issues whatsoever.
