DI water - de-ionized: contains no cations or anions (ie Mg2+, Ca2+, Fe3+, Cl-, etc.), can contain uncharged contaminants, won't SNAFU a pH meter calibration
distilled water - re-condensed water vapor: contains very little ions, can contain contaminants with a similar boiling point as H2O
RO water - diffusion through a size-exclusion, semi-permeable membrane: contains slightly more ions and other contaminants in comparison to the other two methods
Honestly, any of these three can be used to calibrate a pH meter; there's no need to be accurate out to the third decimal place with an aquarium.