Can I Use Tap Water in My Reef Tank?
Quick Answer
No — tap water contains chlorine, chloramine, heavy metals, nitrates, phosphates, and silicates that harm corals and promote algae. Always use RO/DI (reverse osmosis/deionization) water for reef tanks.
No — tap water is not safe for reef tanks. While some freshwater aquarists can get away with treated tap water, reef tanks require much purer water. Tap water contains contaminants that stress corals and fuel nuisance algae.
What’s Wrong with Tap Water?
Tap water typically contains:
- Chlorine/Chloramine — Kills beneficial bacteria and harms invertebrates
- Heavy metals — Copper is especially toxic to corals and invertebrates
- Nitrates — Many municipal water supplies contain 10-40 ppm nitrates
- Phosphates — Fuel algae growth and inhibit coral calcification
- Silicates — Promote diatom blooms (brown algae)
- Total dissolved solids (TDS) — Variable mineral content affects water chemistry
Why RO/DI Water is Essential
A RO/DI system removes 99%+ of contaminants through reverse osmosis and deionization. This gives you a blank slate to create perfect saltwater by adding only the minerals you choose via reef salt.
Benefits of RO/DI water:
- Consistent water quality every time
- No unexpected algae outbreaks from hidden phosphates
- Better coral growth and coloration
- Easier to maintain stable parameters
- No chlorine/chloramine concerns
Getting RO/DI Water
You have two main options:
- Buy your own RO/DI unit — $100-300 investment that pays for itself quickly if you do regular water changes
- Buy from your local fish store (LFS) — Most shops sell RO/DI water for $0.50-1.00 per gallon
Testing Your Water
If you must know what’s in your tap water, test for:
- TDS (should be 0 for RO/DI water)
- Nitrates
- Phosphates
- Copper (especially important)
Track your source water quality in the ReefBay app alongside your tank parameters.
What About Dechlorinators?
Products like Prime or Seachem Safe neutralize chlorine and chloramine, but they don’t remove nitrates, phosphates, silicates, or heavy metals. Dechlorinators are not a substitute for RO/DI water in reef tanks.
Bottom line: RO/DI water is non-negotiable for successful reef keeping. It’s one of the best investments you can make for your tank’s long-term health.
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