The short answer
Only if you actually need it. RO (reverse osmosis) water is stripped of nearly all minerals, giving you a blank slate to build the exact water your fish want. That’s brilliant for soft-water species and shrimp — but pure RO must never go straight into a tank. With almost no GH or KH, it has no buffer, so its pH is wildly unstable and it starves plants and invertebrates. Remineralise it or blend it with tap water first. If your tap water already suits your fish, you may not need RO at all.
When RO is worth it
RO earns its keep when your tap water fights you: very hard water for soft-water fish, unpredictable supply, high nitrate or phosphate from the tap, or sensitive shrimp and blackwater species that need precise, low-mineral conditions. It hands you full control over GH, KH and TDS.
How to use it properly
Never use RO neat. Either:
- Remineralise it with a GH+/KH+ powder to your target hardness (best for shrimp and repeatable results), or
- Blend it with tap water — e.g. 50/50 — to roughly halve your hardness while keeping some buffer.
Test the finished mix with a liquid test kit before it goes in, and always add a water conditioner to any tap-water portion.
The honest verdict
For most beginners keeping hardy community fish, good dechlorinated tap water is simpler and just as healthy. Reach for RO when your livestock genuinely need soft water you can’t get from the tap. See softening water, is rainwater safe, and is tap water safe. More in the water testing hub.