That is pretty dangerous advice. Fish will be fine at those levels, but corals are not good osmoregulators. Prolonged shifts in salinity, as low as 10%, from their natural levels have been show repeatedly to result in significantly increased mortality. Corals come from different areas, with different salinity, however, the most common and average for the world is about 35ppt (or s.g. = 1.0264). So, 10% lower than that is 31.5ppt (1.0238 ). Anything under that, long term is potentially quite dangerous to our corals and other invertebrates.