What Is Coral and How Do You Choose It for a Saltwater Aquarium?

Coral is a living marine animal, not a plant or a decoration, and choosing it well comes down to three things: matching the coral type to your lighting and water flow, buying aquacultured specimens where possible, and placing it where it can stay stable. Soft corals are the usual starting point for a new saltwater tank because they tolerate a wider range of conditions than most hard corals. This guide covers what coral actually is, how soft and hard corals differ, what a tank needs to support them, and how to select, acclimate, and place your first specimen.

What coral actually is

A coral is a colony of small animals called polyps. Each polyp has a mouth surrounded by tentacles, and most reef-building corals host microscopic algae (zooxanthellae) inside their tissue. Those algae photosynthesise and supply the coral with much of its energy, which is why lighting is not optional for most species — it is part of how the animal eats.

Two consequences follow from this:

• Coral is livestock. It can grow, reproduce, get sick, and die. It needs feeding (via light, captured food, or both) and stable water.
• Coral is not a decoration. Unlike plastic plants or resin ornaments, it reacts to its environment. Changes in light, temperature, or water chemistry show up as colour loss, closed polyps, or tissue recession.

Corals also build a skeleton. Soft corals support themselves with tiny internal spikes called sclerites and fleshy tissue; hard corals deposit calcium carbonate, which is what forms reef structures.

Soft corals vs hard corals

The practical split for a beginner is soft versus hard (stony), because the two groups place different demands on the tank.

Soft corals are not "easy" in an absolute sense — they still need correct salinity, temperature, and alkalinity — but they generally recover from small mistakes that would kill a stony coral. If your tank is new and still settling, start soft.

What a tank needs before coral goes in

Coral should go into a tank that has already cycled and held stable parameters for a period, not into a freshly mixed system. The key requirements:

Lighting

Match the light to the coral, not the other way round. Soft corals generally do well under moderate reef lighting; SPS corals typically need high output and a spectrum suited to photosynthesis. If you buy a demanding coral first and then try to upgrade the light around it, you will usually lose the coral.

Water flow

Flow carries food and oxygen to polyps and removes waste. It should be indirect and varied rather than a single jet blasting one spot. Soft corals often prefer gentler, alternating flow; many stony corals want stronger current.

Water parameters

Coral keeping depends on stability more than on any single "perfect" number. The parameters to monitor and keep steady include:

• Salinity — kept stable, checked with a calibrated refractometer
• Temperature — stable, within the range your livestock was kept at
• Alkalinity, calcium, magnesium — the building blocks for skeletal growth; more critical for stony corals
• Nitrate and phosphate — present but controlled; zero nutrients can starve corals as surely as high ones stress them
• pH — stable across the day/night cycle

Test regularly and change one thing at a time. Chasing numbers with rapid corrections causes more coral loss than slightly imperfect but stable water.

Placement

Before buying, decide where the coral will sit: how much light reaches that spot, how much flow passes it, and how much room it has to grow. Soft corals can spread and overgrow neighbours, so leave space. Some species are aggressive toward neighbours chemically or by direct overgrowth.

Aquacultured vs wild-collected coral

Aquacultured (tank-raised) coral is grown in captivity, often on real rock, rather than harvested from a reef. For a beginner, tank-raised specimens are usually the better choice because:

• They are already adapted to aquarium conditions — artificial lighting, captive water chemistry, and tank flow — so they typically acclimatise with less shock.
• They are generally hardier and more consistent in appearance than wild-collected pieces.
• They reduce pressure on wild reefs, and many suppliers, such as AquaCorals, specialise in tank-raised soft corals grown on real rock.

Wild-collected coral can be perfectly healthy, but it has to adjust from reef conditions to your tank, which is a bigger transition and a bigger risk for a first coral.

Selecting, acclimating, and placing your first coral

Step 1 — Check the specimen before you buy

Look for open polyps, intact tissue with no white patches or exposed skeleton, and colour that matches the species. Avoid anything with slime, a foul smell, or recession at the base.

Step 2 — Confirm it fits your tank

Ask what lighting and flow the coral was kept under, and compare that with your setup. A coral kept under high light will struggle if placed under weak light, and vice versa.

Step 3 — Acclimate

Float the bag to equalise temperature, then drip-acclimate or use the supplier's recommended method so the coral adjusts to your salinity and pH gradually. Rushing this step is a common cause of a coral that never opens.

Step 4 — Place it, then leave it

Set the coral on or attach it to rock in the spot you planned, with the flow and light it needs. Then leave it alone. Corals often take days to open fully after a move; repeatedly repositioning them adds stress.

Step 5 — Observe and adjust slowly

Watch polyp extension and colour over the first weeks. If something looks wrong, change one variable at a time and give it time to respond.

Common problems and how to troubleshoot them

The general rule for all of these: correct the cause slowly. Rapid fixes — a sudden light change or a large water chemistry correction — usually make things worse.

Making the decision

If you are setting up a first saltwater tank, start with aquacultured soft corals, confirm your lighting and flow match the species before buying, and place them with room to grow. Move to hard corals once your parameters have been stable for a sustained period and you are comfortable testing and adjusting. The coral you choose should fit the tank you already have, not the tank you plan to build later.