The Reef Aquarium After Hours: Four Reasons to Keep That Flashlight Handy

Long tentacle anemone (Macrodactyla doreensis) at night under LED lighting

Long tentacle anemone (Macrodactyla doreensis) at night under LED lighting

Anyone who has done any night diving knows that nighttime activity on coral reefs is very, very different from what goes on there during the day. A dramatic “changing of the guard” occurs, with diurnal fish seeking refuge in the reef and nocturnal predators and planktivores taking dominion. Parrotfishes, wedged in caves or crevices, cloak their chemical signatures in mucous cocoons. Morays, only partially visible during the day, slither from their dens and swim in the open in search of prey. Octopuses, too, lose their daytime shyness and ply the reefs with busy tentacles. Night-feeding corals that appear bare and lifeless while the sun is up come into their glory with polyps fully extended. Bioluminescent organisms of all kinds put on otherworldly displays. And that’s just scratching the surface!

In the closed system of a marine aquarium, the transition from day to night might not be quite as profound as it is on the coral reefs, but there’s still a lot going on in your tank at night that warrants at least an occasional peek. Here, in no particular order, are four reasons you might want to keep a flashlight close at hand:

1. To see what’s wigglin’ in the water column

I’m always surprised that the water column in my tank, which is essentially devoid of non-fish life during the day (at least the kind visible to the naked eye), hosts so many free-swimming critters after dark. Just last night, I shined a small LED flashlight through the tank and spotted five or six inch-long worms wiggling rapidly and frenetically through the water column. In fact, you may see all kinds worms, various larvae/medusae, and other microfauna wriggling or pulsing their way around the tank at night. (Come to think of it, if slimy, wiggly things give you the heebie-jeebies, maybe this isn’t such a good idea after all!)

2. To take stock of your “pod” population

Many fish benefit from having a healthy population of amphipods, copepods, and other tiny crustaceans (e.g. mysids) they can feast on. Some fish, such as mandarins and other dragonets, really shouldn’t even be attempted unless these microfauna are available in abundance.

So how can you tell where your pod population stands? Scan the tank with a flashlight at night. During the day, you’ll discern the presence of pods only by the odd individual scurrying from hiding place to hiding place or the occasional molted exoskeleton floating in the water column. But at night, these little buggers emerge from hiding and swarm on the rocks and tank walls.

3. To spot troublemaking hitchhikers

Think no potentially predacious crabs or shrimps have taken up residence in your tank? Don’t be so sure! Carefully check the tank with a flashlight well after dark—when they tend to emerge from hiding and wreak havoc—and you may be surprised to discover that a troublemaking nocturnal crustacean has been living right under your nose all along. I’ve discovered many a stowaway crab in this manner.

4. To catch stinging corals in the act

As alluded in my opening paragraph, some corals keep their tentacles to themselves during the day and expand them at night for feeding and self defense. In the case of corals that produce elongated “sweeper” tentacles or are merely placed too close to neighboring invertebrates, this can result in “unexplained” damage to other specimens that manifests during the day.


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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.


  1. The reefs also become covered in urchins at night. They are all over the place sometimes so numerous that you can’t see the rocks. That’s also why we don’t see any algae on natural reefs. It is there or those urchins would be sending out for pizza.

  2. I am always surprised how many bristle worms I have. They crawl all over the place at night. I must have hundreds if not more. Some quite big like 5″ or so. I think most of the life in my tank is hidden during the day.

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