Starting a reef tank: the chemistry-first checklist

Tank and stand selection

Bigger is more forgiving. The math is unromantic: a parameter swing in 20 gallons is double the parameter swing in 40 gallons given the same disturbance. If you can fit a 75-gallon, build a 75-gallon. If you can fit a 120-gallon, build a 120-gallon. The water volume is what buys you time when something goes wrong, and something always goes wrong.

Standard rectangular dimensions (48×24×24 for a 120, 36×18×18 for a 50) hit a lighting and flow sweet spot that bowfronts and odd shapes don't. Cubes are fine. Anything with a footprint where width is less than depth is a flow-and-lighting puzzle you don't need.

Get a real stand. A wood stand built for the tank weight, or a steel stand from a fabricator, or the manufacturer's match — not a converted dresser. Use the aquarium weight calculator to confirm: a 120-gallon reef tank, fully stocked with rock and sand, lands at around 1,400 pounds. A floor joist running perpendicular to the stand can handle that. Parallel to the joists, you want to be over a load-bearing wall.

Rock and sand

Dry rock is the default now. Real Reef, Marco, CaribSea LifeRock — all fine. Skip pukani if you can't cure it for a month; it leaches phosphate for months otherwise. Plan on 1 to 1.5 pounds of rock per gallon of display, less if you're building negative space (which you should be — coral grows, the rockscape gets denser, not sparser).

Sand is a religious argument I won't litigate. The functional answer: CaribSea Special Grade aragonite (1–2 mm) is the right grain size for most builds — fine enough for sand-sifting gobies, coarse enough that it doesn't blow around in moderate flow. Plan on a 1-inch bed in the display. Anaerobic deep sand beds (4+ inches) work but they're a separate hobby with separate failure modes. Bare bottom is also valid and dramatically easier to maintain — you just lose the goby option.

Salt and RO/DI

You need a four-stage (sediment, carbon block, RO membrane, DI resin) reverse-osmosis unit producing 0 ppm TDS water. Tap water has chloramine, phosphate, copper from pipes — any of which will torpedo a reef tank. A BRS or SpectraPure 4- or 5-stage unit runs $150–$250 and pays for itself in dead-coral avoidance within a year.

For salt: pick one and stop changing it. Tropic Marin Pro Reef, Red Sea Coral Pro, Fritz RPM, Instant Ocean Reef Crystals — the differences between current name-brand salts are smaller than the test-kit variability between Salifert and Hanna. The one rule: don't use a freshwater-tank salt or an aquaculture salt for a reef. Mix to 35 ppt / 1.0264 specific gravity at 25 °C, which Randy Holmes-Farley defends as the natural seawater target in his specific gravity article.

Use a refractometer calibrated with 35 ppt calibration fluid. Hydrometers are temperature-sensitive plastic toys and they drift. Spend the $30 on a refractometer and a $10 bottle of cal fluid. The salinity calculator handles the temperature corrections if you do use a hydrometer.

The cycle

Add rock, sand, saltwater. Add a bottle of beneficial bacteria (Dr. Tim's One and Only, Fritz TurboStart 900, Bio-Spira — any of them). Add ammonia source — Dr. Tim's ammonium chloride, or a small pinch of fish food, to bring ammonia to 2 ppm. Test daily.

You're looking for: ammonia drops to 0, nitrite spikes then drops to 0, nitrate appears (10–40 ppm is normal at end-of-cycle). This takes 10–28 days with bacteria added; 4–6 weeks without. When 2 ppm of dosed ammonia becomes undetectable within 24 hours, the cycle is done.

Do a 30% water change. Now your nitrate is low, your alkalinity is back to salt-mix spec, and you have a working biological filter. Now you can start adding livestock — not before, regardless of how cycled your bacteria-in-a-bottle promises it is.

Lighting

LEDs have won. AI Hydra, Radion XR, Reefi, Noopsyche — all credible. You want roughly 1 watt of modern LED per gallon for an SPS-capable build, less for a softie-only tank. For a 120-gallon, that's two Hydra 64s or two Radion XR30s, roughly. PAR meters (Apogee MQ-510 is the consumer standard) confirm what your eye can't.

Run a 6–8 hour photoperiod with 1–2 hours of ramp on either end. Acclimate any new corals to the upper-tank PAR over weeks, not days. Most coral deaths in well-run tanks are light-burn from a too-eager new owner setting LEDs to 100% on day one.

Flow

Target 20–40× total tank turnover per hour at the powerhead level, which is separate from your return pump turnover. A 120-gallon wants 2,400–4,800 gph of in-tank flow from wavemakers. Two AI Nero 5s, two MP40s, or four Jebao SOW-8s — all hit that band.

The goal isn't straight-line current; it's chaotic, multidirectional, gyre-style flow. Powerheads aimed at each other generate the gentle turbulence coral wants. A single jet pointed at one wall makes detritus piles and bored fish.

First livestock

First fish should be hardy, peaceful, and disposable in the emotional sense — meaning if it dies you learn what went wrong without grieving. Yellowtail damsel, ocellaris clownfish, royal gramma, springer's damsel. Quarantine in a separate tank for 30 days minimum with a copper or formalin treatment. Saying "I'll skip QT this once" is how 100% of reef-tank ich outbreaks start.

Stocking density for marine: WetWebMedia's old stocking guidance approximates 1 inch of adult fish length per 5 gallons of water for a heavily filtered reef tank. The stocking calculator uses this baseline. Reality: target the lower bound for your first year, leave room for the rarefied fish you'll inevitably want later.

Testing cadence

Week 1–4 of a cycle: ammonia, nitrite, nitrate daily. Once livestock arrives: alkalinity twice a week, calcium and magnesium weekly, phosphate weekly, nitrate weekly, salinity weekly. Hanna alkalinity checker, Hanna ULR phosphate checker (HI774), Salifert calcium and magnesium. Drop the Red Sea kits for the daily numbers — you'll have less argument with yourself about what color teal you're looking at.

Once a month, run an ICP test (Triton, ATI, or Oceamo) — it catches trace-element issues no consumer kit can. Triton's standard panel is the most widely cross-referenced.

First coral

Wait three months from the day the cycle finished. Not the day you added water — the day ammonia hit zero. Three months. The biofilm and mineral chemistry stabilize on a timescale that has nothing to do with what your test kits report. Hobbyists who add SPS at week 6 are the same hobbyists who post "my tank crashed" threads at week 12.

Start with soft corals — green star polyp, zoanthids, mushrooms. They tolerate parameter drift while you learn what your tank actually does between water changes. After two months of softies thriving, add LPS — frogspawn, hammers, torch. After two more months of healthy LPS, add the first SPS frag and watch it for six weeks before adding another.

This is slow. The hobbyists who go slow are the hobbyists who post pictures of mature tanks at year five. The hobbyists who go fast are the hobbyists who sell their setup on Craigslist at year two.

What not to buy

Skip: copper-based ich treatments in the display tank (kill anything inverts), "reef booster" additives that don't list ingredients, UV sterilizers as a beginner spend (useful later, not now), automatic feeders for the first six months (overfeeding is the most common beginner failure and an autofeeder amplifies it), and any "all-in-one" trace dosing product before you have ICP data to know what your tank actually lacks.

Buy: a refractometer with calibration fluid, a Hanna alkalinity checker, a quarantine tank (a $30 plastic tub with a sponge filter), a backup heater, two thermometers, and a notebook. Document what you change and when. Memory lies; notes don't.

What day 90 looks like

Three months in, a competently run reef tank has: stable salinity at 35 ppt, alkalinity at 8–9 dKH steady, calcium at 420 ppm, magnesium at 1,350 ppm, nitrate around 5 ppm, phosphate around 0.04 ppm, two or three healthy fish, half a dozen softie colonies, and zero algae issues. There's a thin film of coralline starting on the back glass. Your dosing pump runs four times a day. You spend twenty minutes a day on it.

That's the goal. Not a tank packed full of acros at month three — a quiet, stable, boring tank at month three. The interesting tank is the one at month thirty.