RO Water & Remineralisation: A Practical Guide for Aquariums
When to use RO or DI water, how to remineralise for fish and shrimp, target GH/KH/TDS ranges, and common mistakes with Equilibrium-style products.
RO water & remineralisation
Reverse osmosis (RO) and deionisation (DI) strip almost everything from tap water — chlorine, nitrate, phosphate, and hardness. That purity is powerful for soft-water fish, Caridina shrimp, and precise planted tanks, but zero-mineral water is not safe for fish until you remineralise it correctly.
When RO/DI makes sense
- Hard tap water incompatible with your target species (e.g. soft-water discus, Caridina shrimp)
- Elevated nitrate or phosphate in tap water that fuels algae despite good tank maintenance
- TDS creep in shrimp tanks where you need predictable parameters
- Mixing with tap to dilute hardness without guessing — e.g. 50% RO + 50% tap
When tap water is fine
Many community tanks thrive on dechlorinated tap water alone. If your tap matches your fish and plants, RO adds cost and complexity without benefit. Test GH, KH, and pH before investing in an RO unit.
Remineralising after RO
Pure RO water has GH ≈ 0 and KH ≈ 0. Fish and shrimp need minerals for osmoregulation and molting; plants need magnesium and calcium.
Products
| Product type | What it raises | Typical use |
|---|---|---|
| GH booster (e.g. Equilibrium-style) | Calcium + magnesium | Soft-water fish, planted tanks |
| KH booster / alkalinity | Carbonates | pH stability |
| Shrimp mineral (e.g. Salty Shrimp) | GH + KH + trace elements | Caridina and Neocaridina to spec |
| Custom mix | Variable | Advanced keepers with target recipes |
Process
- Fill container with RO water at the temperature you will add to the tank
- Dose remineraliser gradually — never dump powder directly into the display tank
- Stir well and wait several minutes for full dissolution
- Test GH, KH, and TDS before use — see TDS guide
- Adjust in small steps; easier to add than remove
Target ranges (examples)
| Livestock | Typical GH | Notes |
|---|---|---|
| Neocaridina cherry shrimp | 6–10 dGH | TDS often 200–300 ppm |
| Caridina bee shrimp | 4–6 dGH | Low KH; species-specific |
| Discus / soft tetras | 3–6 dGH | Stable low KH |
| Livebearers / Rift cichlids | 10–20+ dGH | May use tap or heavy remineralisation |
Always confirm against your species profile — ranges vary.
RO system basics
- RO membrane removes ~95–99% of dissolved solids
- DI resin polishes remaining ions — used for ultra-pure water or final stage
- Waste water — RO produces 3–4 gallons of waste per gallon of product; plan drainage
- Storage — use food-safe containers; stagnant RO can pick up CO₂ and drop pH
- TDS meter — essential for monitoring membrane performance; see TDS meter hardware
Common mistakes
- Adding RO straight to tank without remineralising — osmotic shock and pH crash
- Only raising GH and ignoring KH — unstable pH swings
- Chasing TDS alone without testing GH/KH — TDS is a sum, not a recipe
- Inconsistent mixing — different buckets each water change stress shrimp
- Old membranes — rising product TDS means the membrane needs replacing
Frequently asked questions
Do I actually need RO water?
Only if your tap water fights what you are keeping. Hard tap suits livebearers and rift lake cichlids perfectly well. RO earns its place for soft-water species, shrimp, and reef tanks, or where your supply carries nitrate, phosphate or silicate you cannot otherwise remove.
What happens if I use pure RO without remineralising?
RO has almost no mineral content and no buffering capacity, so pH becomes unstable and swings sharply. Fish cannot osmoregulate properly in it and invertebrates cannot build shells. Pure RO is a starting point, never a finished water.
What GH and KH should I aim for?
Broadly: 4–8 GH and 3–5 KH suits most community tanks; Caridina shrimp want lower, around 4–6 GH and 0–1 KH; Neocaridina prefer 6–8 GH with some KH. Match the species rather than a general target, and change it gradually.
