Why Reef Tanks Need RO or RO/DI Water

Tap water can carry nitrate, phosphate, silicate, and metals that fuel algae and irritate sensitive corals. RO/DI units strip most dissolved solids; a TDS meter on the output confirms the filters are still effective — replace cartridges when TDS creeps up.

Use RO/DI for top-off and for mixing saltwater. Match batch salinity before water changes. Store mixed water in a clean barrel with a heater and powerhead if you mix large volumes.

If RO is not available short term, some keepers use distilled water from the store — expensive at scale but better than untreated tap for a reef. Note your water source in ReefDiary tank settings for future troubleshooting.