Introduction
I’m not sure how you feel about it, but whenever I imagine the ‘ideal’ reef aquarium, I conjure a vision in my mind of a bountiful, reef tank chock full of corals. The hypothetical aquarium is an aquatic cornucopia with SPS, LPS, leather coral, mushrooms, and more all living together, filling nearly every square inch of rock work, cascading in all directions. That vision has been ingrained in my mind since the first day I decided “I have to have a reef tank.” Do you share my vision?
In fact, I have seen pictures of a few tanks that look like that. Almost every ‘tank of the week’ that you will see on other aquarium websites looks like that. I want my tank to look like that. But it doesn’t.
A trip to the local public aquarium
The reason I’m writing about this now is that I had a bit of an epiphany this weekend related to my previous image of what the ideal tank should look like. I took my kids to the Adventure Aquarium, a public aquarium in Camden, NJ. While my kids enjoyed a display that let them crawl under and pop their heads up into a ‘bubble’ inside the tank, I took the opportunity to photograph an adjacent coral display.
Among the electric-looking brain corals and dinner plate-sized hairy mushrooms, I noticed a humble-looking favid species tucked in between. It didn’t stand out for its colors, but its peaceful appearance caught my eye. I snapped a photo, not realizing at the time that this coral was engaged in a silent battle.
Favia coral sweeper tentacles
When I got home and reviewed my photos, I was surprised to see that the ‘peaceful’ favid was anything but. Extending from a polyp near the base were the long, thin profiles of several menacing coral sweeper tentacles.
Favia coral Sweeper tentacle not causing any harm
This wasn’t the first time I had seen coral sweeper tentacles, but it was jarring to realize that this coral was fighting for its place while I was enjoying the view, oblivious to the struggle.
What is a coral sweeper tentacle?
A coral sweeper tentacle is a specially adapted appendage that certain coral species have (especially Large Polyp Stony Corals). These tentacles are generally several times longer than the coral’s feeding tentacles and are armed with potent nematocysts capable of delivering a strong sting.
We call them “Sweepers” because the corals seem to send them out, sweeping back and forth in search of something to sting. Corals use sweeper tentacles to damage or kill nearby rivals, securing their own place on the reef and preventing being out-competed by neighbors.
What triggers sweeper tentacle deployment?
Sweeper tentacles are not always present; their extension is typically a response to environmental cues and competition. The main triggers include:
- Proximity to other corals: When another coral is within reach, especially if it is a different species, many corals will deploy sweepers to defend their territory.
- Competition for space or light: Crowded conditions or shading from neighboring colonies can prompt aggressive behavior.
- Changes in water chemistry: Sudden shifts in parameters such as alkalinity, calcium, temperature, or nutrients can stress corals and change normal polyp or tentacle extension. That response is not, by itself, proof of aggression.
- Physical stress or injury: Damage from handling or tank maintenance can sometimes trigger defensive responses.
Not all corals are equally prone to this behavior. Genera such as Favia, Euphyllia, Galaxea, and Hydnophora are especially notorious for deploying long, aggressive sweeper tentacles, while many soft corals and some SPS species rarely do so. Sweeper tentacle deployment is often a temporary response to perceived threats or competition, not a constant state.
A stress-relieving hobby for us but the coral sweeper tentacles were signs of a potentially stressful situation for them
The realization I had was this: while this hobby may be a stress-relieving, peaceful activity for us—every day is still a life and death battle for the invertebrates in our care. Corals don’t know they are ‘supposed to’ be tranquil. They simply carry out the actions coded for in their genes. Sometimes, that program says… “begin a war with the coral next to you. Take it out at all costs.”
Why do corals behave this way? In the wild, space and resources on the reef are limited. Over millions of years, corals have evolved aggressive mechanisms—like sweeper tentacles—to compete for territory, light, and nutrients. These behaviors aren’t random or malicious; they’re survival strategies that help corals secure their place in a crowded environment. Understanding this natural competition allows us to work with, rather than against, coral biology in our tanks.
Please forgive the anthropomorphic liberties I took there, but I think you get the point. As much as we may want them to be, our tanks may not be the placid paradise we want them to be for our corals. And while the natural coral reef environment may look like an underwater city with coral ‘skyscrapers’ racing to the surface and occupying every possible millimeter of the substrate, the reality is that what looks like a seemingly placid underwater scene could be characterized as a violent battle (in some cases) fought on a coral-by-coral basis.
What lessons can be gleaned from this?
So maybe the ideal tank shouldn’t be crammed to the brim with different colonies. Maybe the ‘tank of the week’ shouldn’t be so loaded that any minor blip in water parameters is likely to cause a catastrophic cascade of events that causes a significant die-off.
Corals in our stewardship are battling for their lives every day. Can we agree, as hobbyists, to try to take that into consideration when we plan out our tanks?
Recognizing coral aggression and its consequences
Coral aggression isn’t always obvious at first glance. Typical signs include:
- Tissue recession: A coral suddenly losing tissue on the side facing a neighbor may be suffering from a sting.
- Localized tissue loss: Damage concentrated on the side facing a nearby colony can point to contact or stinging, but it is not diagnostic on its own. Check water quality, pests, disease, shading, and physical injury before assigning the cause.
- Visible stings or necrosis: White, dead patches or stringy tissue can indicate recent nematocyst contact.
Unchecked aggression can damage neighboring colonies and create tissue that must be removed before it decays. In a small or already stressed system, substantial tissue loss can add to nutrient and infection pressure. It’s important to distinguish aggression from other causes of decline—look for patterns, proximity, and physical evidence of stinging, then verify the rest of the tank’s conditions.
I hope this doesn’t come across as too preachy. But that simple little sweeper tentacle really gave me pause, and I was wondering if anyone else out there felt the same way.
Remember to give your corals enough space
In our home reef tanks, problems often arise when we over-crowd. It can be hard to leave space between two frags, but we have to allow for them to grow out and fill in the available space.
How much space is enough?
There is no universal spacing number for every coral. Sweeper reach varies by species, colony size, flow, time of day, and individual behavior, and some taxa can reach far beyond the distance that looks safe in daylight. Use these placement principles instead of relying on a fixed inch rule:
- Research the exact coral: Do not treat Favia-type corals, Galaxea, Euphyllia, Lobophyllia, soft corals, and zoanthids as if they share the same reach or mode of competition.
- Plan for the longest observed reach: Inspect after lights-out and under normal flow, then leave additional room for growth. Galaxea and other long-sweeper corals may require substantially more clearance than a generic LPS estimate suggests.
- Use placement and barriers: Put aggressive colonies where their downstream sweep does not cross a neighbor, and use separate rockwork when practical. Remember that barriers do not prevent allelopathic or chemical competition.
Anticipate future growth by researching each species’ mature size and growth pattern. When in doubt, err on the side of more space and keep monitoring—corals can always be moved closer later, but separating them after aggression occurs is much harder.
Balancing aesthetics and coral health
A densely packed look is appealing, but comes with increased risk of aggression and maintenance challenges. Consider starting with wider spacing and letting colonies grow in naturally. If you want a lush appearance, group compatible species together and use rockwork to create natural barriers.
Conclusion
Am I being too sappy, or do we owe it to the invertebrates in our tank to provide enough space for them to coexist without provoking them to initiate war on their coral neighbors?
The next time you find yourself daydreaming about what new frag you want to glue into that 1 x 1 inch bit of space between your frogspawn and your zoanthid polyps—try to remember this article—and that one little coral sweeper tentacle in the image above. And let’s band together and vote for the tank of the week…that gives each colony enough space to grow without being attacked by the other corals around it.