Water Is Everything

A reef tank is not a container of water with animals in it. It's a container of chemistry with animals in it — and every drop you add either upholds that chemistry or undermines it. Most reef problems that get blamed on lights, on fish stores, on bad luck, trace back to water. So before we mix our first batch of ocean, let's understand what we're actually making.

Why Tap Water Sabotages Reef Tanks

Your tap water is safe for you. It is not safe for a reef, and the reasons are specific:

• Phosphate and nitrate. Municipal water routinely carries both — trivially low for drinking, but a continuous fertilizer drip for algae in a closed system. Tanks topped off with tap water fight algae forever and never know why.
• Silicates feed diatoms, the brown dusting algae that plagues new tanks (you'll meet it in the uglies chapter). Tap water keeps refueling it.
• Chlorine and chloramine are added precisely because they kill microorganisms — including the nitrifying bacteria your tank depends on. Dechlorinators neutralize them, but treat only these, not the rest of the list.
• Copper leaches from household plumbing. Fish tolerate traces; corals and invertebrates are famously copper-sensitive. It accumulates in a tank; it never leaves on its own.
• Inconsistency. Even good tap water changes seasonally as your utility adjusts treatment. A reef is a stability project; an input that changes without notice is a saboteur.

Can some people run reefs on tap water? A few do, in a few lucky municipalities. But you can't know if you're lucky until the tank tells you — slowly, expensively, in algae and mystery losses. Purify instead.

RO/DI: How Pure Water Is Made

The hobby standard is a 4-stage RO/DI system — reverse osmosis plus deionization — a set of cartridges that plumbs to a faucet or washing-machine line and turns tap water into effectively pure H2O. The stages, in order:

1. Sediment filter catches particles — rust, grit — protecting everything downstream.
2. Carbon block adsorbs chlorine and chloramine, which would otherwise chew up the RO membrane.
3. RO membrane — the workhorse. Water is forced through a membrane so fine it rejects ~95–98% of everything dissolved: phosphate, silicate, copper, the lot. The rejected water exits as waste (typically 3–4 gallons per purified gallon; it's fine for houseplants).
4. DI resin polishes off the last few percent, exchanging remaining ions until the water reads 0 TDS — zero total dissolved solids.

That number matters, so measure it: an inline TDS meter (most units include one) should read 0 on the output. When it starts creeping to 1, 2, 5 — your DI resin is exhausted and it's time to replace it. Cartridges are consumables: sediment and carbon every ~6 months, DI resin as TDS dictates, membrane every few years. Total ongoing cost is small, and the unit pays for itself against buying water within a year.

The alternative: most fish stores sell RO/DI by the gallon. This is a legitimate way to start — many reefers run nano tanks on store water for years. Bring your TDS meter the first time; a store that sells water reading above 0 is telling you something about their maintenance. Jugs get heavy and errands get old, which is why most people eventually buy the unit. Either path is fine. Tap water is not.

Salt Mix: Engineering Seawater in a Bucket

Natural seawater is a solution of some 70+ elements in remarkably fixed proportions. A modern reef salt mix replicates it — sodium and chloride, of course, but critically also the reef-building trio of calcium (~420 ppm), magnesium (~1350 ppm), and carbonate alkalinity (~8–9 dKH) in correct ratios. This is why you buy reef salt rather than, say, water-softener salt or 'marine' salt from questionable sources: the minor elements are the entire point.

Which brand? Editorially: it matters less than forums suggest. Every major reef salt supports a thriving beginner tank. What matters is picking one and staying with it, because switching brands shifts your baseline chemistry — and stability beats any particular number. Buy the size that stays fresh (an opened box absorbs humidity and clumps; keep it sealed in a bin) and check the label's mixed parameters once with your test kits so you know your baseline.

Mixing: The Ritual

You'll do this hundreds of times, so let's get the habits right from batch one.

1. Start with RO/DI water in a dedicated food-safe container — a 5-gallon bucket or a lidded brute-style can. Salt water only, forever; never one that's held soap.
2. Add salt to water (not water to salt — pouring water onto a salt bed creates concentrated puddles that can precipitate minerals). Most mixes run roughly half a cup per gallon to reach reef salinity, but measure by reading, not by scoop.
3. Power a small pump or powerhead in the bucket and let it mix until crystal clear — typically 1–4 hours. Add a heater to bring it to tank temperature (77–79°F).
4. Measure salinity and adjust. Low? Sprinkle in more salt, wait, remeasure. High? Add RO/DI. Sneak up on the number.
5. Use it within a couple of days. Freshly mixed salt water is fine to use once clear and temperature-matched; most reefers mix the evening before a water change.

Your target: specific gravity 1.025–1.026, which is 35 parts per thousand (ppt) — real-reef salinity. You'll see old advice quoting 1.020–1.023 for fish-only tanks; ignore it. You're building a reef. Run reef salinity from the start and never think about it again.

Measuring Salinity: Trust, but Calibrate

Salinity is the parameter you'll measure most and the one where cheap tools lie hardest. The swing-arm hydrometer — that plastic box with a pointing needle, bundled with starter kits — is notorious: bubbles cling to the arm, calibration drifts, and two consecutive readings of the same water can disagree by whole points.

The beginner-right tool is a refractometer with calibration fluid. It reads salinity from how dissolved salt bends light through a drop of water on a prism: two drops, close the plate, hold it to any light, read the line. Thirty seconds, repeatable, and about the price of two bags of salt.

One discipline makes it trustworthy: calibrate with 35 ppt calibration fluid, not plain freshwater. Calibrating at zero and reading at 35 lets small scale errors stretch across the range; calibrating at your target pins accuracy exactly where you need it. Check calibration monthly and before any reading that surprises you. A surprising number is a re-test, never an action item — write that on the wall.

(Digital salinity pens exist and are pleasant; they still need calibration solution and fresh batteries. The refractometer is the reliability baseline.)

Top-Off: The Daily Drop of Discipline

One last principle that confuses every newcomer once: water evaporates; salt doesn't. As your tank loses water to evaporation, the salt left behind concentrates and salinity climbs. Therefore:

• Top off evaporation with pure RO/DI water — never salt water.
• Replace water you remove (water changes) with mixed salt water matched to tank salinity and temperature.

Mark your tank's full waterline with a piece of tape and top off to it daily — a mason jar of RO/DI by the stand makes it a ten-second habit. Later, an auto top-off device can do this for you; the habit comes first.

With pure water, honest salt, and a calibrated instrument, you now make better seawater than most of the coastline. Next we put something in it: rock, and the architecture of your reef.

FAQ

Can I use tap water in a saltwater tank?

It's strongly discouraged. Tap water carries phosphate, silicate, chlorine, and often copper — algae fuel and invertebrate toxins that accumulate in a closed system. Use RO/DI water from your own unit or a fish store; it's the single cheapest insurance policy in the hobby.

What salinity should a reef tank be?

35 parts per thousand, or 1.025–1.026 specific gravity — the salinity of a natural reef. Measure with a refractometer calibrated against 35 ppt fluid, not a swing-arm hydrometer, and top off evaporation daily with fresh RO/DI so the number stays put.

How long should salt water mix before using it?

Until it's fully clear and temperature-matched — typically one to four hours with a pump circulating. Many reefers mix the night before a water change for convenience. Avoid letting mixed water sit unheated and uncirculated for many days; mix close to when you'll use it.