Frag Tank: What It Is and How Reefers Use It

Growers isolate coral fragments in a smaller, dedicated saltwater system to multiply colonies, control pests, and trade or sell spares without ever touching the show display. The idea is straightforward: cut a small piece, mount it on a plug, and let it recover in low-stress conditions while the mother colony keeps thriving at home. Most hobbyists run one on a bare bottom with a frag rack, a gentle return pump, and reef-spectrum lighting tuned to the corals they actually keep.

Below, the sections cover what sets this setup apart from quarantine or grow-out systems, when the cost pays off, and how to frag and mount corals without losing a single piece down the overflow.

A Frag Tank Is a Dedicated Propagation System, Not a Spare Tank

Hobbyists use the word “frag” loosely, and that is where most confusion starts. A dedicated propagation system has one job: turn small cuttings into saleable or display-ready colonies. It runs tighter on parameters than a quarantine tank (which exists to observe and medicate sick fish) and finishes faster than a grow-out tank (which holds juveniles for months or years).

How It Differs From Quarantine and Grow-Out Tanks

Quarantine tanks lean on low light, bare-bottom simplicity, and medication dosing for fish, with little concern for coral growth. Grow-out tanks accept larger frags or juvenile colonies and run them for long stretches, often with more aquascape and higher bioload. A propagation system sits between those two: stable parameters, strong lighting, gentle flow, and a layout built around plug spacing rather than rockwork.

The Bare-Bottom, Minimal Aquascape Convention

Most reefers run these setups with no sand and almost no live rock, just a PVC or acrylic frag rack. A bare bottom means you can siphon detritus in seconds, spot pest hitchhikers fast, and rearrange plugs without crashing a micro-reef. The trade-off is reduced biological surface area, which is why a small refugium or a simple sponge filter is so common.

Size, Stocking, and Side-Income Potential

Footprints range from 10-gallon nanos to 40-gallon breeders, with most active propagators settling around a 20-long or 30-gallon. Stocking plans usually cap at 20 to 40 plugs at a time so each one gets light and flow. Plenty of hobbyists offset the cost of equipment by trading or selling frags locally and to stores, turning a hobby expense into a small side income.

That side-income angle is real, yet most beginners should weigh whether their goals justify the setup before buying anything.

Do You Actually Need One? Honest Cases For and Against

Adding another system just because your favorite YouTuber runs one is a fast way to burn money. The real question is whether your reef, your stock list, and your free time justify the maintenance load of a second saltwater setup.

When It Becomes Essential

Fast-growing SPS like Acropora will carpet your rockwork in months, sting neighboring corals, and force constant pruning. A propagation system gives those trimmings a home where they can encrust without competing for light. The same applies to large Euphyllia colonies, active zoanthid gardens, and any hobbyist who wants to trade or sell extras instead of tossing them.

When It Is Unnecessary

A single-soft-coral nano with a few mushrooms, leathers, and a hammer coral rarely needs a second tank. Stable parameters, low growth rates, and no trading plans mean there is nothing to frag in the first place. Forcing a propagation system into that picture adds water changes, testing, and equipment costs for zero return.

The Most Common Mistake

Running a small system with unstable parameters does more harm than not fragging at all. Calcium, alkalinity, and magnesium swing fast in low-volume water, and a struggling frag rarely recovers. Test kits like the API Reef Master Test Kit make tracking these values realistic on a weekend schedule, but only if the numbers drive dosing decisions.

Can a Sump Substitute?

For very small operations, a sump or back compartment can host a handful of plugs on a small rack. It is not ideal, since light and flow are limited, but it works for low-light softies. Anything beyond a few frags at a time deserves its own dedicated footprint.

Choosing Tank Size, Footprint, and Placement

Size drives every other decision, from lighting cost to how often you haul buckets. Pick the smallest footprint that still gives you room to grow, because a cramped system stresses both you and the corals.

Small, Medium, and Grow-Out Footprints

A 10 to 20-gallon tank suits hobbyists with a handful of frags at a time and fits on a counter or sturdy shelf. A 20 to 40-gallon breeder gives active propagators room for multiple vertical racks, which is where bulk frag production really happens. Anything past 40 gallons starts behaving like a small display, with more water volume to heat, light, and clean.

Why Surface Area Beats Depth

Coral frags need light and gentle flow more than they need depth. A shallow, wide tank spreads plugs out so each one gets even PAR coverage from a single fixture. A tall, narrow footprint forces you to stack racks vertically, which is fine for SPS but a pain for LPS that prefer horizontal placement.

Placement, Plumbing, and Living-Space Practicalities

Put the system near a water source and a drain so water changes stop feeling like punishment. Keeping it close to the main display shortens the trip for new frags, which reduces stress and temperature shock. Noise matters in living spaces, so a quiet controllable return pump and a small protein skimmer beat loud air-driven setups when the tank sits behind a couch or a bed.

With the footprint settled, every component decision now hinges on how much you’re willing to spend and how hands-on you want to be.

Tank Size	Best For	Typical Frag Count	Footprint Note
10-gallon nano	Beginners, softies	5 to 15 plugs	Fits on a counter or shelf
20-gallon long	Active hobbyists	15 to 30 plugs	Wide, shallow, easy to light
30-gallon breeder	Side propagators	25 to 50 plugs	Supports multiple racks
40-gallon breeder	Small businesses	40 to 80 plugs	Needs a sturdy stand and sump

Equipment, Lighting, Flow, and Filtration by Budget

You can build a working system on a tight budget or kit it out like a mini display, and both will grow coral if the parameters stay stable. The trick is matching gear to the corals you keep, not the other way around.

Budget Tier Setup

A clip-on LED, a small hang-on-back filter, an air-driven sponge filter, a heater, and a basic frag rack cover the essentials. The sponge filter alone handles biological filtration for a low-bioload soft-coral system. This setup is fine for zoanthids, mushrooms, and leathers, but struggles with light-hungry Acropora.

Ideal Tier Setup

A dedicated reef-spectrum LED, a controllable return pump, a small protein skimmer, and an optional refugium turn the system into a real production line. The skimmer pulls organics before they fuel algae, and the refugium grows chaeto to outcompete nitrates and phosphates. For SPS-heavy work, an EcoTech Marine Radion or an AquaMaxx fixture delivers the spectrum and intensity that small polyp stony corals need to encrust and color up.

Lighting and Flow by Coral Type

Softies and LPS want moderate light and indirect flow, while SPS demand high PAR and stronger, randomized movement. Gentle, indirect flow keeps frags in place while healing tissue, and a wave maker on a low setting usually beats a single pointed powerhead. Match the spectrum to the coral, and resist cranking intensity until the frag shows clear polyp extension.

Stable Parameters in Small Volumes

Alkalinity, calcium, and magnesium swing harder in small water volumes because each dose represents a bigger percentage of the total. A two-part dosing setup or a small calcium reactor keeps numbers locked in, and testing every few days catches drift before frags start to stress. Trace elements matter too, but only after the big three are stable.

Component	Budget Pick	Ideal Pick
Lighting	Clip-on LED	Reef-spectrum fixture (AquaMaxx, EcoTech)
Filtration	HOB filter + sponge	Protein skimmer + refugium
Return pump	Air-driven or small DC	Controllable DC pump (Trigger Systems)
Rack	Basic egg crate	Acrylic stackable rack (Innovative Marine)

Fragging and Mounting Corals Step by Step

Cutting a coral feels risky the first few times, but the process is mechanical and forgiving once you understand what each species needs. Match the tool to the coral, dip incoming plugs, and give the cut time to heal before pushing light or flow.

Tools by Coral Type

Branching SPS like Acropora snap cleanly with bone cutters or even fingers, while thicker skeletons need a coral saw. LPS like Euphyllia take a clean slice with a razor or scalpel between polyps, and soft corals often break by hand. Keep tools sharp, rinse them between colonies, and avoid sawing back and forth, which crushes tissue and invites infection.

Pre-Frag Preparation

Start with a healthy mother colony under stable parameters, never a stressed or recently moved piece. Dip every incoming plug in a coral dip from Two Little Fishies or a similar product to kill flatworms, nudibranchs, and eggs. Have plugs, epoxy, and a towel laid out before the first cut, because mid-cut fumbling is how frags end up on the floor.

Mounting Options

Ceramic frag plugs with a small epoxy dab are the workhorse, cheap, reusable, and easy to space on a rack. Magnetic mounts let you pull a frag out for a quick photo or sale without lifting the rack. For LPS and softies, mounting directly on rubble rock with epoxy gives the colony a head start, since encrusting growth begins the moment the frag touches the substrate.

Difficulty Ratings by Common Coral

Zoanthids and mushrooms sit at the easy end, since a clean cut and a plug are usually all it takes. Euphyllia and Acropora fall in the intermediate range, demanding cleaner cuts and tighter parameter control. Trachyphyllia and other fleshy LPS rank as advanced, because tissue damage and infection are real risks without a sharp scalpel and iodine dips.

Post-Frag Care

Keep the freshly cut frag in low light and low flow for the first 24 to 72 hours so tissue can recede and seal without burning. An iodine-based recovery dip reduces bacterial load on the wound. Watch for tissue recession, polyp bailout, or rapid color loss, all of which signal that parameters slipped or the cut was too aggressive.

Even a clean cut can go sideways once the frag hits the main display, where competition and pests introduce a whole new set of failure modes.

“Heads up: frags with exposed skeleton often look rough for the first week. Resist the urge to move them or dose heavy medications; stable parameters and clean water do more than any additive.

Placement, Pests, Algae, and Common Failure Modes

Most frag losses come down to placement, pests, or parameter swings, not bad luck. A consistent routine and a few habits will solve nearly every problem before it starts.

Vertical Rack Strategy

Stackable racks multiply surface area without expanding the footprint, and they keep each frag at a different height so sweeper tentacles do not reach their neighbors. Keep at least an inch of spacing between plugs to stop a dying frag from melting onto a healthy neighbor. Place light-loving SPS on the top tiers and shade-tolerant LPS or softies lower down to match each coral’s needs.

Algae Control

Algae explodes in small systems with strong light and excess nutrients, which is a near-unavoidable combo. Manual removal during water changes keeps the bare bottom clean, and a strict photoperiod of 8 to 10 hours prevents runaway growth. A small refugium with chaeto exports nutrients faster than water changes alone, especially in heavier frag tanks.

Pest Prevention

Every incoming frag, whether from a local reefer, a store, or an online vendor, goes through a coral dip before it touches the system. Quarantining new arrivals in a small separate bin for a week catches flatworms, nudibranchs, and aiptasia that would otherwise hitchhike into the rack. Skipping the dip is how entire systems get wiped out by a single careless trade.

Troubleshooting Melting, RTN, and Polyp Bailout

Rapid tissue necrosis (RTN) and slow tissue necrosis (STN) both show up as receding tissue exposing white skeleton, and both usually point to alkalinity swings or bacterial infection. Polyp bailout, where a coral detaches from its skeleton and drifts around the tank, almost always traces back to a parameter crash or a hostile cutting job. Detached plugs near the overflow are a separate emergency, so any loose plug should be pulled from the rack and re-secured the moment it shows movement.

When Frags Are Ready to Move to the Display

A frag is display-ready when you see encrusting growth along the plug base, stable color that matches the mother colony, and at least two to four weeks of consistent polyp extension. Frags rushed into a display too early often bleach, recede, or get outcompeted by established neighbors. Holding them a few extra weeks in the propagation system almost always produces a stronger, more colorful addition to the main reef.

Final Thoughts

One small coral colony can become dozens of healthy frags, filling trading lists, sale tanks, and new displays without crowding the main reef. Treat it as a focused propagation system with stable parameters, gentle flow, and a clean bare bottom, and it will pay for itself in a few large trades alone. Skip it if your stocking list and goals do not need it, but once you start trimming fast-growing SPS, a dedicated setup stops feeling optional.

FAQ