Can I drink the RO reject water?

Technically — yes, it's safe. But it contains a concentrate of all the minerals and chlorine from the tap. Most people use it to water plants or for cleaning.

RO+DI vs 4-stage RO — which is better?

For aquariums, 4 stages are enough (output TDS 5–15 ppm). DI brings it to 0 ppm — needed only for display caridina, overkill for most.

Deep-diveAdvanced9 min readMay 29, 2026

RO water and remineralization: when you need it and how

When to buy a reverse-osmosis unit, how to remineralize with salts, and how to avoid the classic mistakes.

An RO (reverse osmosis) unit is the most expensive piece of kit, yet sometimes the only option for certain fish and shrimp. This article shows when RO is truly needed and when it's wasted money.

What an RO filter does

A semipermeable membrane passes water molecules but holds back practically everything dissolved: salts, minerals, chlorine, heavy metals, pesticides. Output TDS is 0–10 ppm vs 200–800 ppm from the tap.

When you need RO

• Discus, apistogramma, tetras kept biotope-style (need GH 1–4) • Caridina (crystals, blue bolt) — mandatory • Ultra-acidic water dwellers (wild-type bettas, chocolate gourami) • Hard tap water (>15 °dGH) + plants that want soft

When you don't

• Neocaridina, guppies, platies, mollies — they love hard water • African Malawi/Tanganyika cichlids — want hard • Community tanks with neon/cardinal tetras when tap is GH 4–12 • Mid-demand plants (anubias, vallisneria, crypt)

Remineralization

Pure RO water without minerals is dangerous — fish suffer osmotic shock. Always add mineral salts before topping up. To make soft GH 4–6 water, either mix 30–50 % RO with tap, or use dedicated salts.

Remineralizing salt brands

• Salty Shrimp Bee Mineral GH+ — for caridina (GH only, no kH) • Salty Shrimp GH/KH+ — for neocaridina and community tanks • Seachem Equilibrium — budget option for plants • Don't use sodium-based products — that's marine salt, not aquarium remineralizer

Step-by-step procedure

1. Drain RO water into a bucket. 2. Add the mineral mix by label (typically 1–2 g per 10 L). 3. Stir, let dissolve for 30 minutes. 4. Measure TDS, pH, GH. Targets depend on the fish. 5. Only after measurements — pour into the tank.

Costs

RO unit USD 50–100 (4-stage), membrane USD 25–50 replaced every 2 years. Salts USD 10–30 for 6 months. TDS meter USD 10. Total: USD 100–200 start, USD 20–50/year ongoing.

RO without a TDS meter is blind driving. Buy the TDS meter first — it's the control instrument for the whole system.

FAQ

Can I drink the RO reject water?: Technically — yes, it's safe. But it contains a concentrate of all the minerals and chlorine from the tap. Most people use it to water plants or for cleaning.
RO+DI vs 4-stage RO — which is better?: For aquariums, 4 stages are enough (output TDS 5–15 ppm). DI brings it to 0 ppm — needed only for display caridina, overkill for most.

AuthorGoldie Editorial

Goldie editorial team

Practising aquarists with a combined 30+ years of experience · Biologists and editors, fact-checking against FishBase and Seriously Fish · Every piece is reviewed by a qualified ichthyologist before publication

Reviewed byAlex Koval

Senior aquarist, breeder, show judge

27+ years in aquaristics · Certified IAPLC judge (International Aquatic Plants Layout Contest) · Registered breeder of an Apistogramma agassizii line

Sources

Salty Shrimp — RO and remineralization guide · Salty Shrimp · 2026-05-29
Seachem — Equilibrium reference · Seachem · 2026-05-29