Short answer: RO (Reverse Osmosis) water isn’t automatically “better,” but it can be better depending on your fish, your tap water, and your goals. For many aquariums, simply dechlorinating tap water is perfectly fine. For others, RO (or RO/DI) water gives you control you can’t get any other way.
Let’s break it down clearly and practically.
When RO Water Is Better
RO water is essentially a blank slate—very low in minerals, no chlorine/chloramine, no heavy metals, no nitrates, no phosphates. That makes it ideal when:
1. Your tap water is problematic
- Very high GH/KH
- Very high pH
- High nitrate straight from the tap
- Heavy metals or contaminants
- Water softeners (which add sodium)
If your tap water is like this, RO gives you consistency and avoids fighting your water every week.
2. You keep fish that require soft, acidic water
Examples:
- Discus
- German rams
- Wild-caught tetras
- Apistogramma
- Caridina shrimp
These species often thrive in RO + remineralizer because you can dial in exact GH/KH.
3. You want total control for breeding
Breeders love RO because:
- You can set GH/KH precisely
- You can match rainwater conditions
- Avoid tap-water swings
When Dechlorinated Tap Water Is Perfectly Fine
Most community tanks do great with tap water as long as it’s not extreme.
Tap water is usually better for:
- Livebearers (guppies, mollies, platies)
- African cichlids
- Goldfish
- Most hardy community fish (danios, barbs, gouramis)
These species like minerals. RO water without remineralization would actually be harmful.
The Big Catch with RO Water
RO water must be remineralized before use.
Pure RO has:
- GH = 0
- KH = 0
- TDS ≈ 0
That means:
- pH becomes unstable
- Fish can suffer osmotic stress
- Plants struggle
- Beneficial bacteria weaken
So you need a product like:
- Seachem Equilibrium (GH)
- Salty Shrimp GH/KH+
- Seachem Alkaline/Acid Buffer (KH/pH control)
This adds cost and complexity
The Practical Rule of Thumb
Here’s the simplest way to decide:
| Your Tap Water | Best Choice |
|---|---|
| Stable, moderate GH/KH, low nitrate | Dechlorinated tap water |
| Very hard or very soft | RO + remineralizer |
| High nitrate or contaminants | RO |
| Keeping soft-water specialists | RO |
| Keeping hard-water fish | Tap water |
My take, based on your methodical style
Since you’re a careful, research-driven aquarist, RO water gives you precision, and precision is powerful. But it’s only worth the effort if your tap water is working against you or your fish need specialized conditions.