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Saltwater fundamentals

Reef tank basics: live rock, salinity, and protein skimming

Three things set a reef apart: what seeds the biofilter, how precisely you measure the water, and how waste leaves before it becomes nitrate.

2 min readUpdated September 25, 2026By FishicianPublished August 27, 2026How we source

The short answer

  • Live rock is scenery and filter. It carries much of the biological filtration.
  • Uncured rock needs its own cycle. Its die-off causes an ammonia and nitrite spike. No fish until tests clear.
  • Measure salinity with calibrated tools. Clean and verify the instrument as its manufacturer directs; bubbles, residue and calibration errors can affect readings.
  • A skimmer exports waste as foam before it becomes nitrate. It supplements water changes; it doesn't replace them.
  • Stability earns the next step. Additions should depend on stable conditions and the needs of the species.
A home reef aquarium with coral colonies, rockwork and fish
A home mixed reef, photographed in May 2020. Corals add care requirements beyond keeping fish alone.Photo: Nano reef aquarium, May 2020 — Treetopz · CC BY-SA 4.0. Resized for display.
Build a reef in this order
  1. Cycle and verifyLive rock and/or controlled ammonia
  2. StabilizeSalinity, temperature and flow
  3. Hardy livestockQuarantined first
  4. Sensitive speciesOnly after a long stable stretch

Live rock

  • Cured rock has finished its die-off; uncured carries more life and more decaying material.
  • With uncured rock, treat the start like a fishless cycle: test, run circulation and the skimmer. See cycling.
  • Rinse rock in mixed saltwater, not tap water, before placing it.

Salinity

  • Follow the salt-mix instructions for mixing and temperature before measuring; undissolved salt can give an unrepresentative sample.
  • Match temperature and salinity before any water goes into the display. See water changes.
  • Top off evaporation with fresh water; more saltwater raises salinity.

Protein skimmer

  • Air bubbles collect dissolved organics and lift them out as foam into a cup.
  • Expect a break-in of days to weeks with thin or patchy foam.
  • Empty and rinse the cup regularly so it doesn't overflow or clog.
  • Size it to the real waste load, not the tank's label volume.

Lighting

  • Many corals and anemones host photosynthetic symbionts; others do not. Lighting and feeding must match the species.
  • Fish-only with live rock has far more lighting latitude than a coral reef.
  • Research each coral's needs; one setting doesn't suit everything. See lighting.

Measuring tools and quarantine

  • Calibrate according to the exact instrument’s instructions. Some refractometers require distilled or deionized water; others use a specified standard.
  • Keep hydrometers clean, remove trapped bubbles and follow their temperature instructions. Compare suspect readings with a verified instrument.
  • Check the instrument before acting on a surprising salinity reading.
  • Many marine treatments kill invertebrates, so quarantine new fish before they enter a reef.

Sources

Each source is listed with the part of this guide it supports.