Water parameters

Using Reverse Osmosis Water in Your Aquarium

When Pure Water Makes All the Difference

Reverse osmosis aquarium water station with buckets, remineralizer, and TDS meter

Introduction

Reverse osmosis (RO) water is water that has been forced through a semi-permeable membrane under pressure, removing virtually all dissolved minerals, metals, chlorine, chloramines, and organic compounds. The result is nearly pure H2O with a TDS of 0-10 ppm and essentially no hardness or buffering capacity.

For most fishkeepers with moderate tap water, RO is unnecessary. But for anyone keeping soft-water or blackwater species, breeding sensitive fish or shrimp, or dealing with tap water that is excessively hard, heavily chloraminated, or contains elevated heavy metals or nitrates, RO water is a game-changer.

Quick Overview

Best forCaridina shrimp, discus, blackwater tanks, and very hard tap water

RO outputNearly pure water with very low TDS and little buffering

Must doRemineralize before adding to any aquarium

Watch forSlow production rate and reject water waste

Who Actually Needs RO Water?

Caridina shrimp keepers: Crystal Red, Crystal Black, Blue Bolt, and other caridina varieties require very soft, acidic water (TDS 100-150 ppm, GH 4-6) that most tap water cannot provide; RO remineralized with caridina-specific mineral supplements is the standard
Discus keepers: discus from wild Amazonian stock do best in very soft, warm water; breeding discus almost always requires RO
Blackwater biotope enthusiasts: very low TDS (50-80 ppm) water accurately replicates Rio Negro conditions; impossible to achieve with most tap water
Hobbyists with very hard tap water: mixing RO with hard tap water allows precise control of final hardness; easier and more reliable than chemical softening
Areas with high nitrate in tap water: some municipal water supplies have tap nitrate levels of 20-40 ppm, making it impossible to keep tank nitrate low; RO solves this immediately

RO Systems: What to Look For

A basic aquarium RO unit consists of a sediment pre-filter, a carbon block filter, and the RO membrane. More advanced units add a deionization (DI) stage that removes the final traces of dissolved solids to produce truly zero-TDS water.

75-100 GPD membrane: gallons per day rating; a 75 GPD unit produces enough water for regular changes on multiple tanks
Sediment filter: removes particulates that would clog the membrane; replace every six months
Carbon block: removes chlorine and chloramines that would destroy the RO membrane; replace every six months or when chlorine is detected post-carbon
DI stage (optional): removes final dissolved solids to 0 ppm; useful for caridina shrimp and reef tanks; DI resin exhausts and must be replaced based on TDS readings

RO water is slow to produce (a 75 GPD unit makes about 3 gallons per hour) and wasteful (for every gallon of RO water produced, 3-4 gallons of reject water go down the drain). Many hobbyists direct the reject water to garden use or outdoor ponds to reduce waste.

Remineralizing RO Water

Pure RO water cannot be used directly in an aquarium. With zero hardness and zero buffering capacity, its pH is unstable, fish cannot maintain proper osmoregulation in it, and the biological filter is less effective without mineral ions. RO water must always be remineralized before use.

For general community fish: Seachem Equilibrium or similar GH/KH powder; target GH 6-10, KH 3-5 for most tropical community tanks
For caridina shrimp: Salty Shrimp GH+ (no KH) or similar caridina-specific mineral mix; target TDS 100-150, GH 4-6, KH near zero
For blackwater setups: a small amount of Seachem Equilibrium for essential minerals, then add tannins via Indian almond leaves and driftwood; target TDS 50-100
Always mix and test remineralized water in a bucket before adding it to the tank; testing directly in the tank after addition is too late to correct an error