Coral Frag Rack Sizing: Magnetic, Rim Hang, and Standoff Frag Plug Holders Explained

A frag rack is a flat plate with a grid of holes that holds coral frag plugs upright in your tank. Two numbers decide which rack you need: the stem diameter of the plugs you already own, and how you want to hold the plate in the tank.

Here is the part nobody says out loud. There is no industry standard size for a frag plug stem. We pulled 14 plug product pages across six brands. They publish four different round stem sizes, from 9 mm up to 11.113 mm, and one brand publishes two of them inside its own catalog. That is why the same bag of plugs can rattle in one rack and jam in the next. So this guide starts with measuring your own plugs, then walks the mount choice.

What is a frag rack, and what does it actually hold?

A frag rack holds frag plugs, not corals. A frag plug is a small ceramic or aragonite piece shaped like a mushroom. It has a flat top disc, called the crown, where the coral gets glued, and a stem underneath. The rack's holes hold the stem.

That split matters more than it sounds. The stem sets the hole size. The crown sets the spacing between holes, because a crown is two to four times wider than its own stem and corals grow out over it. Get those two numbers right and everything else about a rack is preference.

A rack also does one plain job. It keeps small plugs standing in one spot instead of tipping over on the sand where you lose them. That is the whole point of the plate.

Magnetic, rim hang, or standoff: which frag rack mount fits your tank?

Pick the mount by where you want the plate and how often you plan to move it. Three mounts cover almost every rack sold, and they solve different problems.

What size are coral frag plug stems, and how do you measure yours?

Published frag plug stems come in four round sizes: 9 mm, 9.525 mm (3/8 inch), 10 mm, and 11.113 mm (7/16 inch). The 3/8 inch stem is the most common by a wide margin. It shows up on nine product pages across four brands.

You will read all over the hobby that a frag plug stem is 1/2 inch, or 12.7 mm. We went looking for that number and could not find it. Not one of the 14 product pages we pulled states a 1/2 inch round stem. The only published 1/2 inch stem is square, on a single plug, and a round hole wide enough to clear its corners would hold none of the four round sizes.

Where 1/2 inch really does show up is on the rack side. Rack makers write "holds up to .5 inch stem frag plugs", which is a limit on the largest stem the rack accepts. It is not the size of the hole they drilled. No rack maker we checked publishes an actual hole diameter at all.

1. Pull a plug you already own and dry the stem off.
2. Set a caliper across the stem, not the crown. The crown is the wide top disc and it is not what goes in the hole.
3. Measure across the middle of the stem. The very bottom is often rougher and reads wide.
4. Do this on five plugs from the same bag, not one. Ceramic plugs are kiln fired, so they vary from each other.
5. Write down the largest number you get. That is the size your holes have to clear.
6. No caliper? A cheap plastic one is plenty for this. A ruler will not read to a tenth of a millimetre.
7. Own plugs from two brands? Measure both. There is a real chance they are different sizes, because they often are.

Which hole size should you pick?

Match the hole to the stem you measured. We cut each hole 0.60 mm wider than the stem it is meant for, so the plug drops in instead of being forced.

How far apart should frag rack holes be?

Space the holes off the crown, not the stem. The crown is the wide top disc, and it is far wider than the stem underneath it, so two plugs collide at the top long before they touch at the bottom. Corals then grow out over that crown, which only makes the top wider.

Published crowns run from 1/2 inch (12.7 mm) up to 1 1/2 inch (38.1 mm). We add 12.7 mm of gap to the crown width to get the closest spacing we will lay out. That gap is our own choice. No rack maker publishes hole spacing anywhere, so there was nothing to copy.

Do magnets belong in a saltwater tank?

Magnets are fine in a salt tank right up until salt water reaches the magnet itself. Neodymium magnets carry a thin plating. Chip that plating and water gets in, and the magnet swells, rusts, and breaks apart inside your tank.

So the pocket has to be sealed. Ours is printed closed on every side, with the disc fully inside solid plastic. There is no lid, no cap, and no hole for you to push a magnet through later. That also means the disc has to go in while the part is still printing, not afterwards.

Which is exactly why our four magnet mount racks are not for sale right now. We are settling how the magnet gets supplied before we list them. The pocket takes a 20 mm by 5 mm disc. That is a pocket dimension and nothing more. We do not publish a pull strength for it and we are not going to.

The magnet on the outside of the glass is worth a word too. Use a coated or encased pair. Wet hands and salt creep are normal parts of tank work, and a bare disc sitting on the outside of the glass will find both.

If you want a rack today, the rim hook and the standoff have no magnet in them at all.

How many frags fit on a rack, and what size should you buy?

Buy for the plugs you own plus the next six months, not for the plate. We make four sizes: 6 hole, 12 hole, 24 hole, and a 40 hole that ships as two 20 hole plates.

Why the 24 hole rack takes a smaller plug than the 40 hole

That table looks backwards, so here is the reason. Our print plate is 220 mm square, and every part has to fit inside it in one piece.

A single 24 hole plate spaced for a 1 inch crown works out to 216.6 mm wide. That leaves under 2 mm of room on each side, which is a coin flip, not a margin. So the 24 hole is spaced for a 3/4 inch crown instead, at 184.9 mm wide, and we say so on the product.

The 40 hole gets around the same wall by being two separate plates. Each plate is its own print, so each one keeps the full 1 inch spacing. That is why the bigger rack takes the bigger plug.

Where should a frag rack sit in the tank?

Put it where your hand reaches easily and where you can see it without moving rock. That is the practical answer, and it is the one most people get wrong on the first try.

• Somewhere your arm actually reaches. You will be pulling plugs in and out of it constantly.
• Off the path of your magnet glass cleaner, if the rack is on the glass.
• Deep enough that a top off swing never leaves a plug sitting dry.
• Somewhere you can lift the whole plate out, not just the plugs, when it is time to clean it.
• Not right in front of a return or a powerhead, where the plate takes the full push of the water.

Frag rack materials: acrylic, egg crate, rock, and printed PETG

Four materials cover almost every rack sold. They mostly differ in how the holes get made, and in whether you get any say in the hole size.

Why egg crate is worth understanding even if you never buy it

Nearly every plug brand claims its plugs fit egg crate, and egg crate is a real dimensioned product, so it works as a sanity check on the whole size list. A cell opening is 12.7 mm square. A round stem that passes through it cannot be wider than 12.7 mm.

Every published round stem clears that bound by 1.6 mm to 3.7 mm. Two separate lines of evidence, the plug makers and the rack makers, land in the same band. That is a good sign the four sizes above are the real range, even though nobody calls them a standard.

We print in PETG, the material aquarium keepers have used submerged for years. Rinse any printed part in tap water before it goes in the tank. Made in the USA, and printed in the USA when you order.

Frequently asked questions