Why Is My Coral Not Growing?
You did all the right things. The tank is cycled, the livestock has settled in, the corals are open and nicely coloured up. Even months later, that frag still looks like the day you glued it down. No new branches, no encrusting edge, no new polyps. This is one of the most frustrating puzzles in reefkeeping because a coral that is not growing does not cry out for help. It’s just there, quietly giving you a feeling that something is wrong.
The good news is that stalled growth nearly always has a trackable cause. Here’s how to work through it.
First, Is It Actually Not Growing?
Growth rates vary enormously. A healthy Acropora can add a centimetre or two in a month under good conditions, while a brain coral or a slow LPS might barely seem to change over a whole season. Before you start fiddling, take photos from the same angle every couple of weeks and compare them. You'll often find that "no growth" is really "slow growth", especially in a younger system.
Also ask whether it's one coral or all of them. A single stalled colony points to a local problem such as a pest, a neighbour or a bad spot in the tank. Everything stalling at once points to something in the water or the lighting.
Light: Too Little, Too Much, or Wrongly Timed
Most reef corals get a large share of their energy from zooxanthellae, the algae living in their tissue. No usable light means no energy, and no energy means no skeleton building. Weak output, the wrong spectrum or a fixture mounted too high can all leave corals coasting.
But more isn't always better. When intensity passes what a coral can handle, it spends its energy on protecting itself rather than growing. Pale corals, pulled in or sitting in a fading patch under a strong light, may be overexposed rather than underfed.
Photoperiod matters too. A fixed, consistent lighting cycle helps corals keep their daily rhythm, and some reefers have found that a split or erratic schedule interrupts it. Keep the light simple and consistent.
Alkalinity, Calcium and Magnesium
Hard corals build skeletons from calcium carbonate, so these three parameters are the raw materials. The more important lesson, though, is stability. A tank holding a steady 8 dKH will usually outgrow a tank that swings between 7 and 10 because of patchy dosing or a salt mix that's inconsistent from one bucket to the next.
Test alkalinity regularly and, if you dose, automate it so the level stays put. Check magnesium as well. If it runs low, calcium and alkalinity become harder to hold steady, and some hobbyists find growth stalls even when the other numbers look perfect.
Nutrients: Clean Isn't Always Better
This is the one that catches the most people out. Chasing zero nitrate and zero phosphate can starve corals, because the zooxanthellae need a little of both to do their job. Measurable, low-level nutrients, typically a few parts per million of nitrate and a trace of phosphate, suit most mixed reefs.
The flip side matters too. Phosphate that climbs too high can interfere with skeleton formation and lead to algae and cyanobacteria. You're aiming for a sweet spot, and the only way to find it is to test rather than guess. If your corals look pale and growth has slowed in a very clean tank, nutrients are the first thing to check.
Flow: The Invisible Ingredient
Corals sit in a thin layer of still water called the boundary layer. Without enough movement, waste and oxygen build up around the tissue and nutrients struggle to reach it. Flow is a coral's circulatory system, and too little of it can quietly suppress growth.
Watch your corals. SPS should be gently buffeted from several directions, while many LPS prefer softer, less direct movement. If you see detritus settling on tissue, flow is probably too weak. If the polyps are permanently pulled in, it may be too strong.
Trace Elements and Old Systems
In a mature tank that relies only on water changes, trace elements such as strontium can slowly deplete. If you've had the same setup for years and growth has slowly faded, a trace element test and a refreshed routine are worth a look. The same goes for older systems where phosphate has built up in the rock over time, then leached into the water.
Pests, Neighbours and Crowding
Sometimes the answer is biological. Flatworms, nudibranchs and other pests can hide on or beneath a coral and irritate it without killing it. Inspect the base and the underside with a torch, especially on stalled individuals.
Neighbours matter as well. Sweeper tentacles and chemical competition between corals can suppress a colony that's sitting too close to an aggressive species. Give corals room, and think about how much they'll grow into before you place them.
Don't Forget the Basics
Temperature and salinity should be steady. New corals need proper acclimation, and a coral that's just arrived may spend weeks adjusting before it puts energy into growth. Switching salt brands, swapping lights and rearranging rock all add to that stress.
A Simple Troubleshooting Order
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Confirm the problem with dated photos.
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Test the big three: alkalinity, calcium and magnesium, plus salinity and temperature.
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Test nitrate and phosphate and check they aren't sitting at zero or too high.
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Inspect the coral for pests and check what's nearby.
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Review light and flow and adjust gradually.
Change one thing at a time so you know what worked.
Conclusion
A coral that isn't growing is rarely beyond help. Most of the time it's a matter of stability, energy and nutrients, rather than any one dramatic fault. Be patient, test consistently, make small changes, and give each one a few weeks to show. Growth in a reef tank is a slow reward for steady husbandry, and when it finally starts, you'll wonder why you ever worried.
If you are struggling with coral growth or have a coral-related question. Reach out to the experts at Reefco Aquariums for advice.
