Choosing Healthy Coral Frags: A Beginner's Checklist

Picking healthy coral frags is a visual skill, and it is learnable in an afternoon at the rack. Almost everything worth knowing about a frag is visible before you pay for it: how far the polyps come out, whether tissue still covers the base, how long it has clearly been growing on that plug, and what else is living on the plug with it. We buy and place coral for client systems every week, and the frags that settle in fastest are almost always the ones that passed a slow two-minute look under decent light before they went into a bag.

Where healthy coral frags come from matters as much as how they look

Read the system before you read the coral. A frag tank tells you a great deal in ten seconds: nuisance algae fuzzing the racks, cyanobacteria films on the plugs, dead skeletons left in the grid, or half the shipment sitting closed at midday. None of that means every frag in the tank is doomed, but it does tell you how much attention the system gets and how much recovery the coral has had.

Ask how long the frag has been in the building. A colony cut and glued yesterday has not yet shown anyone anything, while a frag that has sat for two or three weeks, opened daily and started to creep onto its plug has already proved something. Ask where in the rack it was kept as well, because a coral grown on the sand bed under moderate light will not enjoy being placed at the top of your tank on day one.

Parameters matter for the same reason. A frag raised at 11 dKH does not enjoy dropping into a display running 7.5, and the sulk that follows gets blamed on the coral rather than the move. Availability shifts week to week, so it is worth calling before you drive out. Our Wheeling shop page explains how pickup works, and a quick call confirms what is actually in the frag tanks that day.

Polyp extension separates healthy coral frags from stressed ones

Extension is the fastest signal a coral gives, and what counts as good depends on the type. Zoanthids should be open with visible skirts rather than pursed shut. Euphyllia should look inflated and full, not stringy or deflated against the skeleton. Acropora should show a soft fuzz of polyps rather than a bare, glassy surface. Mushrooms should be flat and turgid instead of puckered into a ball.

Compare across the rack rather than against a mental picture. If every frag in that tank is half closed at two in the afternoon, the tank is telling you something the individual coral is not. If one frag is closed while its neighbors are open, look closer at that one specifically. Recent handling muddies this badly, so if a coral was lifted, dipped or moved in the last few minutes, ask to come back in ten.

Some signs are not ambiguous. Stringy mesenterial filaments hanging out of an LPS coral, a gaping mouth, heavy slime production or tissue visibly pulling away from skeleton all mean the animal is under real stress right now. That coral may well recover in the store, but it should recover there rather than in your tank. Asking the shop to hold it for a week is a reasonable request and a common one.

Tissue at the base tells you the recent history

Look at the bottom of the frag, not the top. On an SPS frag, tissue should run down to the glue line, and a band of bare white skeleton between the tissue and the plug means recession is in progress. Old, healed skeleton looks dull and often has a little coralline on it. Freshly stripped skeleton is bright white with a sharp edge, and that edge is where the coral is currently losing ground.

Algae is a useful tell at the same boundary. Turf or hair algae growing right up against living tissue is competing with the coral, and once it is established at that margin it usually keeps winning. Similarly, a euphyllia head cut too close to the flesh, or a chalice with a raw, unhealed rim, will spend weeks sealing that wound before it does anything else. Most healthy coral frags show a clean, healed margin instead.

Color needs a skeptical eye, because store lighting is designed to flatter. Heavy blue LEDs make almost anything look spectacular, so ask to see the frag under whiter light or in a photograph taken without a filter. Pale tips on an acro can mean strong light and fast growth or the beginning of bleaching, and the difference is context. A uniformly brown coral has usually been kept too dim rather than being unhealthy.

The plug and the encrusting edge give away a frag's age

Encrusting growth is the single most reassuring thing on a plug. When tissue has crept off the frag and onto the ceramic, the coral has been sitting there long enough to be comfortable, and it has already survived the cut, the glue and the move into a new system. A hard glue bead with no growth over it, by contrast, tells you the frag was mounted very recently and has everything still ahead of it.

Judge the plug itself as well. Plugs covered in hair algae, dotted with vermetid snails or crusted with old coralline arrive carrying all of that into your tank. Many hobbyists cut or snap the frag off the plug entirely before it goes in the display, and that is good practice regardless of how the plug looks. At minimum, scrub the plug and plan to keep the frag in a low-traffic spot until you know it is clean.

Size buys you margin. A two-polyp zoanthid frag has very little room for error, while a six or eight polyp cluster can lose one to a bad week and carry on. The same logic applies to a single-head euphyllia with a solid skeletal base versus one balanced on a sliver. If you are still building a stocking list, our roundup of beginner-friendly corals and their light and flow needs is a good place to start.

Pests you can spot with your own eyes at the counter

Bring a small flashlight and use it. Aiptasia and majano anemones are visible on plugs and in crevices, and one of either is enough to reconsider the purchase. Vermetid snails show as small white tubes trailing a mucus net. Flatworms look like tan ovals gliding over tissue and are easiest to see against a pale plug. Turn the frag over, because the underside of a plug is where most of this hides.

• Aiptasia or majano anemones anywhere on the plug, base or crevices

• Small white calcareous tubes with mucus nets, which mean vermetid snails

• Tan or rust-colored ovals moving slowly across tissue, which suggests flatworms

• Tiny yellow specks scurrying on acropora, the usual sign of red bugs

• White or cream nudibranchs on montipora, plus crescent-shaped egg masses underneath

Even healthy coral frags carry hitchhikers, and no visual inspection catches eggs. Everything that comes into a system we maintain gets dipped and held before it is placed, without exception, and the same rule serves a home tank well. The procedure itself, including solution choice, contact time, agitation and rinsing, is covered step by step in our guide to dipping and inspecting new corals.

Key takeaways

• Buying healthy coral frags is mostly observation: the system, the polyps, the base, the plug and the pests, in that order.

• A frag that has encrusted onto its plug has already proved it can handle being cut, glued and moved.

• Bare white skeleton with a sharp edge at the base means active recession, unlike dull, healed skeleton.

• Store lighting flatters color, so judge under whiter light and treat pale tips as a question rather than an answer.

• Turn every frag over and check the plug underside, then dip and quarantine regardless of what you found.

Frequently asked questions

How long should a new frag take to open up at home?

Most corals look unhappy for a day or two after a move and start opening normally within three to five days. Euphyllia and zoanthids often take a week. If a frag is still closed after ten days, look at flow, light intensity and placement before you assume the coral was a bad one.

Is a frag that is closed in the store always a bad buy?

Not necessarily, but it is a question rather than a bargain. A coral closes for many harmless reasons, including recent handling, a dip that morning or simply the time of day. What matters is whether it opens while you watch the rest of the tank, and whether the shop is willing to hold it until it does.

Should I remove a new frag from its plug?

Usually yes, and sooner rather than later. Plugs hold algae, pest eggs and old coralline, and they take up space in an aquascape. Snap or cut the frag free once it is settled, then glue the tissue-bearing piece directly onto rock. Leave the coral undisturbed for a couple of weeks first if it arrived stressed.

Work with Marine Concepts Inc

If you would rather have somebody else choose, dip and place the coral, we build and look after reef systems throughout Chicago and the suburbs, and you can see the full range of aquarium services we offer. Frags, dry goods, dips and test kits are available for same-day pickup at Marine Concept Supply in Wheeling, IL, where a call ahead confirms what is on the racks.