RO units and reverse osmosis water in aquariums: who actually needs it?
Let’s be blunt: reverse osmosis water is unnecessary for most beginners. An RO unit is only justified if your tap water is very hard or loaded with nitrates, or if you keep soft, acidic-water species: Caridina shrimp, discus, killifish, Amazon biotope tanks. For a standard community tank, conditioned tap water is fine.
What exactly is RO water?
The RO unit forces tap water through an extremely fine membrane that holds back 95 to 98 % of minerals, nitrates, metals and pesticides. Out comes almost pure water: GH close to 0, KH close to 0, minimal conductivity. Careful: this “empty” water is never used neat — it gets remineralized or blended with tap water.
When does an RO unit become useful?
- Very hard tap water (GH above 15, common in limestone regions): impossible to keep soft-water species without diluting it.
- High nitrates from the tap (over 25 mg/L): your water changes pollute instead of cleaning, as your water tests will reveal.
- Demanding species: Caridina, discus, wild apistogrammas, Amazon biotopes.
- Breeding certain species that only spawn in soft, acidic water.
Home RO unit or jerrycans from the pet shop?
An entry-level RO unit costs 40 to 80 €, a serious model 100 to 200 €, plus replacement membranes (20 to 40 € every 1 to 3 years). It wastes 3 to 4 litres of water for every litre produced. The pet shop sells RO water at 0.30 to 0.60 € per litre: for a 20-litre nano tank, the jerrycan makes more sense; from 100 litres of blended water upwards, the unit pays for itself within a year.
How do you remineralize properly?
Use remineralizing salts (10 to 20 € a tub, GH+ types for shrimp) and check with a liquid test or a conductivity meter (15 to 25 €). The simplest method is still blending: for example 50 % RO plus 50 % tap water to halve the hardness. Find all our water-quality guides on the fishkeeping hub.
Frequently asked questions
Can I use demineralized water from the supermarket?
Possible in a pinch, but it’s made for steam irons, with no strict controls for living creatures. Pet-shop RO water is safer and cheaper per litre.
Is rainwater an alternative?
Yes in theory, but it picks up pollutants from roofs and gutters. Reserve it for clean, filtered collection, and always test before use.
Does a domestic water softener do the same job?
No, quite the opposite: it swaps calcium for sodium, which is harmful in an aquarium. Never use salt-softened water for your fish.
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.