Tap Water in a Reef Tank: Why Not, and What to Use
Chloramine, copper and hidden nutrients make Sydney tap water risky for reef tanks. Here's why, and what to use instead — RO/DI explained in plain language.
What's in tap water, and why reef tanks care
One of the most common questions we hear at the counter is whether tap water is okay for a saltwater tank. It seems harmless enough — we drink it every day — but what's safe for people isn't necessarily safe for corals, shrimp and the more sensitive fish.
Here's a plain-language look at what's actually in Sydney tap water, why it causes trouble in marine tanks, and your options for getting clean water into your system — from a full RO/DI unit down to simply buying water by the jerry can.
Chlorine and chloramine
All mains water is disinfected so it's safe to drink. Older setups used plain chlorine, which famously gasses off if you leave a bucket standing overnight. Sydney's supply, however, is treated with chloramine — chlorine bonded to ammonia — precisely because it doesn't break down quickly in the pipe network. That stability is great for drinking water and bad news for aquarists: chloramine will still be there after a day or two of standing, and it's toxic to fish, invertebrates and the beneficial bacteria that run your biological filter. Worse, when chloramine is broken apart it releases ammonia, which then needs dealing with on its own.
Copper from household plumbing
Even where the mains water itself is fine, most Australian homes have copper pipes and copper hot water systems, and small amounts leach into the water — especially from the hot tap or after water has sat in the pipes overnight. Copper is the classic invert killer: levels far too low to bother fish are generally considered lethal to shrimp, snails, crabs and corals over time, and it tends to accumulate in rock and sand where it's very hard to remove later. This is also why we suggest never using equipment or containers that have touched copper-based medications.
Phosphate, nitrate and silicate
Tap water also carries variable amounts of phosphate, nitrate and silicate. None of these is likely to hurt a fish directly at tap levels, but in a reef tank they're algae food. Phosphate and nitrate feed nuisance algae and can irritate corals when they climb, while silicate is the main driver of those brown diatom films that plague new tanks. The frustrating part is the variability — levels differ suburb to suburb and season to season, so a tank that was fine on tap water for months can suddenly start growing hair algae after a change in the supply.
Why dechlorinator alone is a half-measure
A quality water conditioner (Seachem Prime or similar) neutralises chlorine and chloramine and temporarily binds ammonia and heavy metals. That's genuinely useful — every marine keeper should have a bottle on hand for emergencies — but it's worth being clear about what it doesn't do. It doesn't remove anything from the water: bound copper is still in the system, and phosphate, nitrate and silicate pass straight through untouched. For a fish-only tank that can be an acceptable compromise. For a reef tank, where you're trying to keep nutrients low and metals near zero, conditioner on its own is treating the symptom rather than the cause.
RO/DI water, in plain language
The hobby's standard answer is an RO/DI unit — a small filter assembly that connects to a tap or laundry outlet and slowly produces very pure water. The name just describes the stages the water passes through:
- Sediment filter — a simple cartridge that catches rust, grit and particles, protecting everything downstream.
- Carbon block — removes chlorine and chloramine (for chloramine-treated water like Sydney's, a catalytic carbon block is generally recommended, and many units run two carbon stages).
- Reverse osmosis membrane — the heart of the unit. Water is pushed through a membrane so fine that most dissolved substances, copper and nutrients included, can't follow. This stage is slow and produces some waste water, which is normal.
- DI (deionising) resin — a final polishing stage that grabs the last traces the membrane missed.
To check it's all working, hobbyists use a TDS meter — a cheap pen-style tester that reads total dissolved solids in ppm. Sydney tap water commonly reads somewhere in the tens to low hundreds; a healthy RO/DI unit should read 0 TDS at the output. Zero doesn't mean literally perfect water, but it's the practical signal that the unit is doing its job — and a creeping TDS reading is your early warning that the DI resin or membrane needs replacing. You'll find RO/DI units and TDS meters in our equipment range, and many keepers find the one-off cost pays for itself over years of water changes.
No unit? Buy your water instead
You don't have to own an RO/DI unit to keep a reef. We sell RO/DI water and Saltwater by the container, and for smaller tanks — say under 150 litres or so — plenty of hobbyists find buying water easier than making it. Bring food-safe jerry cans or ask what containers are available.
Top-ups vs water changes
One distinction worth understanding: top-up water and water-change water are not the same thing. When water evaporates from your tank, only the fresh water leaves — the salt stays behind — so evaporation is replaced with plain RO/DI water, no salt added. Water changes, on the other hand, use saltwater mixed to match your tank, typically around 1.025–1.026 specific gravity (about 35 ppt). Mixing them up is a classic way to send salinity swinging.
When tap water is arguably tolerable
To be balanced: some hobbyists run fish-only marine tanks on conditioned tap water for years without drama, and the hobby genuinely disagrees on how much it matters in that setting. If you go that route, use a conditioner that's explicitly rated for chloramine, draw from the cold tap after letting it run a moment (hot water systems and stagnant pipes carry more copper), and accept that you may fight algae more than an RO/DI keeper would. The bigger catch is flexibility — copper and nutrients accumulate in rock and sand, so a tank raised on tap water can be a harder place to add inverts or corals later. For anything with a coral or invertebrate in it, most experienced keepers consider RO/DI the safe default.
Quick checklist
- Sydney mains water uses chloramine — it will not gas off overnight like plain chlorine.
- Never assume standing water is safe; treat or purify every litre that goes in.
- For reef tanks, use RO/DI water for both top-ups and salt mixing.
- Check RO/DI output with a TDS meter — aim for 0 ppm, and replace resin when it creeps up.
- Top-ups are plain RO/DI; water changes are salt mix at ~1.025–1.026 SG (35 ppt).
- No unit? Pre-made RO/DI and saltwater from a marine LFS works well.
- Fish-only on conditioned tap water is a judgement call — reef tanks are a different story.
Common mistakes
- Letting tap water "age" overnight and assuming it's safe — that trick worked for chlorine, not chloramine.
- Relying on dechlorinator alone for a reef tank, then wondering where the algae is coming from.
- Using the hot tap to speed up mixing — hot water systems tend to add more copper.
- Topping up evaporation with saltwater, which pushes salinity steadily upward.
- Never testing RO/DI output, so exhausted DI resin goes unnoticed for months.
- Rinsing salt-mix buckets or filter gear in untreated tap water and letting it drip back into the tank — minor, but easy to avoid with a quick RO/DI rinse.
Need a hand?
If you're weighing up an RO/DI unit or just want water sorted for a new tank, drop into the store in Eastwood and we'll talk you through the options — or browse our live stock and gear online while you're planning.