RO/DI Water Explained: Why Reef Keepers Use It

Reverse osmosis deionization (RO/DI) systems are considered essential by most serious reef keepers, and increasingly common in freshwater planted tanks too.

What’s Actually in Tap Water

Tap water often contains phosphate, nitrate, silicates, copper, and other trace contaminants, any of which can fuel nuisance algae or stress sensitive coral and shrimp, even at levels considered safe for human consumption.

How RO/DI Works

A reverse osmosis membrane removes the majority of dissolved solids, while a final deionization stage polishes the water to near-zero total dissolved solids (TDS), giving you a blank slate to remineralize with salt mix or fertilizers.

Do You Need It?

For reef tanks, RO/DI water is close to essential for controlling algae and protecting sensitive coral, as covered in our Complete Guide to Coral Reef Tanks. Freshwater community tanks can often get by with tap water plus a dechlorinator, though very sensitive species like shrimp benefit from RO/DI too.

Remineralizing RO/DI Water

Pure RO/DI water lacks the minerals fish and invertebrates need, so it must always be remineralized with an appropriate salt mix (saltwater) or mineral supplement (freshwater) before use.