Coral Frag Plugs Explained: Types, Sizes, and How to Mount Coral Frags

A coral frag plug is a small ceramic or plastic disc used to mount coral fragments (frags) while they attach and heal. Standard frag plugs measure 25mm in diameter with a 6mm post and fit most frag racks, tiles, and reef tank substrate. Coral frags typically bond fully to a plug within 2 to 4 weeks under stable conditions.

Whether you are fragging your own colonies or receiving mail-order corals, understanding plug types, mounting techniques, and proper placement will dramatically improve survival rates for new additions to your reef.

Ceramic vs. Plastic Frag Plugs

Ceramic Frag Plugs

• pH buffering: Calcium-based ceramic provides a mild pH buffer and releases trace minerals that support coral encrustation.
• Weight: Heavier plugs (8-12 grams) stay in place on rockwork without additional adhesive.
• Coral acceptance: The porous surface encourages coralline algae growth within 1-2 weeks, helping frags bond faster.
• Drawback: Rough surfaces on low-quality plugs can abrade delicate tissue, especially on zoanthids or mushrooms.

Plastic Frag Plugs

• Cost: Roughly 30-50% cheaper than ceramic, ideal for large-scale propagation.
• Surface: Smooth finish reduces tissue damage, particularly for LPS corals with fleshy polyps.
• Weight: Lighter (3-5 grams), which can be a disadvantage in high-flow tanks.
• Longevity: Plastic does not degrade, but corals may take slightly longer to encrust onto the non-porous surface.

For most hobbyists, ceramic plugs suit SPS corals best, while plastic works well for softies and LPS. The Seatorch FP Series frag plugs use a smooth-finished ceramic design that combines buffering benefits with a polished surface to prevent tissue abrasion.

How to Attach a Coral Frag to a Plug

Cyanoacrylate Gel (Reef Glue)

Best for small frags under 15mm. Apply a pea-sized drop of gel to the plug, press the frag base firmly for 10-15 seconds, and submerge. The glue cures in approximately 30 seconds underwater. Always use gel formula rather than liquid, which runs off before bonding. White residue around the joint is normal and will be covered by coral growth within 1-2 weeks.

Epoxy Putty

Ideal for larger frags over 15mm or irregularly shaped pieces. Knead equal parts resin and hardener for 60 seconds, mold a small ball around the plug post, and press the frag into the putty. Epoxy sets in 5-10 minutes and fully cures within 24 hours. For maximum hold, use a combination: glue the base first, then reinforce with an epoxy collar around the joint.

Placement and Acclimation for New Coral Frags

Light Acclimation

Start new frags at approximately 50% of their target PAR value. If the coral's ideal range is 250-350 PAR, place it initially in a zone receiving 125-175 PAR. Over 10-14 days, increase light intensity by 10-15% every 3-4 days until the frag reaches its permanent position. This prevents photoshock, which can trigger bleaching in even hardy species.

Flow and Quarantine

Position new frags in moderate, indirect flow. Direct laminar flow from a powerhead can stress tissue and dislodge unsecured plugs. Before placing any new frag in your display tank, dip it in a coral dip solution for 5-10 minutes to remove hitchhikers such as flatworms, red bugs, and nudibranchs. Observe the frag for 2 weeks, inspecting the plug and base daily for pests, tissue recession, or rapid tissue necrosis.

Securing Plugs to Rockwork

Once a frag is acclimated, drill a 6mm hole in the rock using a masonry bit, or bond the plug with reef-safe epoxy. Some hobbyists remove the plug entirely once the coral has encrusted, trimming the post flush before epoxying the frag directly to rock.

Frequently Asked Questions

How long does it take for a coral frag to attach to a plug?

Most frags begin encrusting within 1-2 weeks. Full attachment typically takes 2-4 weeks for SPS corals and 3-6 weeks for LPS. Maintaining stable alkalinity between 7.5-8.5 dKH and calcium at 400-440 ppm accelerates bonding.

Can I glue coral frags directly to live rock without a plug?

Yes, using cyanoacrylate gel or epoxy. However, plugs allow easy repositioning during acclimation. If the coral shows stress, you can simply move the plug rather than chiseling a glued frag off the rock.

What size frag plug should I use?

The standard 25mm plug works for most frags. For very small frags under 8mm, use a mini plug (15-18mm). For large colonies over 40mm, use a 35-40mm disc or frag tile for a wider, more stable base.

Do I need to cure ceramic frag plugs before use?

It is good practice to soak ceramic plugs in RO/DI saltwater for 24-48 hours. This rinses manufacturing dust, saturates the porous ceramic evenly, and ensures the plug does not cause a localized pH spike near the frag.

Why is my coral frag not attaching to the plug?

Common causes include unstable alkalinity, excessive direct flow, or algae growth on the plug surface preventing coral contact. Clean the plug with a soft brush, keep alkalinity steady within 0.3 dKH daily variance, and reduce direct flow. Some species like Acanthastrea can take 6-8 weeks to begin encrusting.