Garden Eel Care: Heteroconger hassi

Garden eels take their name from their habit of living in colonies among seagrass beds on the seafloor. The majority of the eel’s body remains embedded in their den, leaving only the head and upper body exposed to snatch particles of food from the water current. And when they sense an approaching threat (or a harmless diver with a camera), they quickly retreat tail-first into that burrow to hide out until they feel safe again.

Striking leopard spots cover the garden eel, with two or three large black patches standing out near the head. They have fantastic eyesight (as many frustrated photographers will attest) in their enormous yellow eyes. And as with many other burrowing fish, you’ll notice an enlarged, slightly upturned mouth.

If you come across a juvenile garden eel, you won’t see those characteristic spots, though. Youngsters are a solid black. This provides a level of protection, particularly as they remain pelagic until they’re large enough to start burrows of their own. That’s when the color breaks up into the spots aquarists have come to recognize.

A tropical species, you’ll find garden eels throughout the Pacific and into the Red Sea. They cluster along the fringes of coral reefs, where the sand allows the best terrain for building a den. And with their quick reflexes, they manage to elude most predators (in addition to tourists).

The average spotted garden eel will live for 35-40 years. And – provided you’re able to mimic the environment of these unique fish – you can expect to see the same lifespan from your eels. However, they’re demanding fish and NOT recommended for beginning aquarists. And even those with some experience under their belts may struggle to get everything perfect.

Divers have glimpsed garden eels at depths between 23-150 feet (7-45m). They prefer the sandy regions extending out from the borders of coral reefs throughout the tropical areas of the Pacific. But they also love seagrass beds, where they have a chance to blend in among the waving fronds. The strong currents in these areas bring food particles directly to them, allowing the fish to spend the majority of their lives safe and secure in their burrows.

With strong muscles, they create a rigid digging structure from their tails. Driving into the sand, they use their dorsal fins to sweep the loose particles up and out of the den. Special mucus glands in their skin then go to work, secreting a compound that cements the burrow walls into a rigid structure. Garden eels use their dens to escape from threats (genuine or perceived) and to sleep in at night.

You need to allow for at least 8 inches (20cm) of sand at the bottom of your reef tank. In an ideal world, you’d go for 12 inches (30cm) and allow your eels to stretch to their full length, but that’s usually out of the reach of most aquarists. (Not to mention it often means a giant wall of sand against the glass – super attractive) And that sand? It needs to be CLEAR. Every garden eel pair needs an entire square foot (0.09m²) of open sand. That means no live rock or coral cluttering up space (though you’ll want to include both for a proper garden eel environment).

Creating a slope with the substrate will help you avoid a pile of sand at the front of the tank. Your eel will still have room to construct a burrow, but you’ll get a more pleasing view. It means finding the right mixture of sand, coral rubble, and crushed coral, though. That way, you won’t have constant erosion happening from your filters and pumps.

As to other décor? Garden eels would prefer a “less is more” approach. They don’t leave their burrows, so there’s no need to clutter the tank with unnecessary accessories. Feel free to add live rock and coral to the perimeter (away from the open sand), but don’t overdo it. You’ll want to respect the open area in the center of the marine tank as much as possible.

Garden Eel Tank Size

While spotted garden eels aren’t active and WON’T swim around, they put size demands on aquarists. You need to have room for that cushy layer of sand in the bottom. But you also have to make sure each of your eels has room to construct their burrow. And when you start doing that math? It goes up.

On average, a saltwater aquarium of 125-180 gallons (473-681L) is ideal. This will make it easiest for you to feed, manage water quality, and allow your colony room to avoid quarrels.

However, you DO have alternatives. Cube tanks work well whenever you’re dealing with a burrowing fish. If you have room for a tank with 24-inch (61cm) sides, you can handle a smaller colony. You won’t have much room for corals, but it cuts down on the space you need – not to mention the costs of purchasing a more extensive aquarium setup.

Are Garden Eels Reef-Safe?

Garden eels are one of the best reef-safe fish you can ask for. As they don’t leave their burrows, you don’t need to worry about them harassing or damaging your corals. And while they ARE carnivores, their tiny mouths aren’t capable of feeding on more than the particles they snatch out of the water column.

If you’re worried about the excavations of the den kicking up too much sand, lay down coarser sand as the top layer in your reef tank. The heavier grain will stay in place better, preventing sand from scattering into the water, where it may drift into your coral polyps. This trick also comes in handy with the recommended feeding practices for these burrowing eels.

Garden Eel Diet

You’ve deduced how to set up the perfect tank for your eels. You don’t even mind that most of your reef tank will feature open sand, with the occasional spotted head poking out. It didn’t seem like THAT much of a challenge. But one of the things that makes garden eels so difficult is feeding them.

They’re carnivores that pluck plankton straight from the water column. And they do this all day, CONSTANTLY. When divers come across them in the wild, they’ll notice the entire colony facing the same direction. The eels turn into the current, where the water brings their plankton favorites from the moment they wake up until they settle in for the night. Dropping food into your aquarium once or twice a day? That isn’t going to cut it.

You need to create an automatic feeding system that cycles food past their burrows throughout the day. This way, they can replicate the same feeding habits they’d see in their natural habitat. One of the easiest ways to accomplish this is with an automatic feeder and several circulation pumps. With the proper positioning, the pumps circulate the food in a lazy pattern past the dens. Then your eels can pick up pieces at their leisure.

And their favorite foods to add to that feeder?

• Fish eggs
• Oyster eggs
• Prawn eggs
• Red cyclops

You can easily purchase frozen options from your fish retailer of choice. And adding a small block to your feeder should cover your garden eels for the day. You can always adjust the amount if you feel you see too much waste. Or you can choose their tank mates accordingly to help with some of the clean-up process.

Garden eels are social fish. While you won’t find two in a single burrow, you WILL see over a hundred eels in a colony along the seafloor. They define their individual “territories,” and the occasional spat breaks out if two males dig dens too close to one another. (Or they may fight if it’s spawning season and they’ve picked the same female) Otherwise, colonies spread out, usually around the largest male.

They retreat into their burrows when the light fades, sleeping during the evening hours. And when dawn breaks, they poke their heads back out of the opening to begin the vital task of eating. Any time someone senses a predator (or very dangerous tourists with cameras), they dive tail-first back into the safety of their home. Rarely will you see a garden eel leave its burrow and swim freely. If you do, it’s a sign of stress. (And you’ll want a sturdy lid on your aquarium, as they CAN propel themselves out of a tank)

Due to the need to keep open sand – and the best manner of feeding garden eels – some of the best tank mates for these fish are sand-dwellers. Not only will the invertebrates keep the substrate looking fresh, but they’ll also remove any of the leftovers your eels happen to miss. Suitable choices include:

• Cerith snails
• Hermit crabs
• Medusa worms
• Nassarius snails
• Sea cucumbers
• Spaghetti worms

Fast-swimming and large fish can pose a threat to garden eels. Easily spooked, they’ll dive into their burrows if they feel threatened – even by peaceful fish. When they’re in their dens, they’re not eating. And without a proper intake of food, they’ll weaken and eventually starve. While your garden eels may learn to adjust to the presence of darting fish, you’ll run a risk with their health getting to that point.

Garden eels don’t like leaving their burrows for anything – even reproduction. Instead, they prefer to stretch across the sand to one another. It’s easy to tell the difference between two males fighting over burrows and a spawning pair, too, as males and females are sexually dimorphic. Males grow larger, with a heftier jaw that juts out further.

When a male chooses a female during spawning season, he fights with any other male showing interest in her. This is when you may see aggression in your eels, especially if their burrows are close together. When she accepts him, the two will stretch across to one another, wrapping their bodies around in an embrace. They may remain entwined for several hours.

The fertilized eggs are released into the current to develop in the open water. The eggs remain close to the surface. After hatching, the juveniles stay pelagic until they’re large enough (and strong enough) to dig a burrow. It’s this free-swimming period that makes them challenging to breed in captivity. You may need to set up a separate tank to make the juveniles feel comfortable until they’re ready to join the adults in the colony.

There’s something hypnotic about a swaying colony of garden eels. However, they’re tricky for aquarists to manage. And before you decide to invest in your first school, you’ll want to think through the pros and cons:

Pros

• Garden eels don’t require much décor in their marine tanks. In fact, the less you have on the sandy substrate, the better.
• These fish are 100% reef-safe, feeding on the same plankton that your corals enjoy.
• When the lights go off for the night, the eels return to their burrow. So you get to watch all of their activity, without a need to worry about what may happen overnight.

Cons

• Garden eels aren’t simple to breed in captivity, and they DON’T handle the stress of shipping very well.
• You can’t get away with an occasional feeding schedule. You’ll need to set up automatic feeders and circulation pumps to provide a constant food source.
• Even peaceful tank mates may lead to stress in these eels if they dart around the reef tank too often.

Everyone’s fascinated by spotted garden eels. They perform an enchanting, swaying dance as they snatch plankton from the current. And if you’re interested in adding them to your collection? You may want more information.

This YouTube video shows garden eels doing what they do best – feeding!

Want to learn more about the best garden eel tank mates?

Do you think these burrowers might be too much to handle? You can always consider another burrowing fish:

Would you believe spotted garden eels are SO shy they can end up impacted by global events? True story! During the 2020 pandemic, aquariums noticed their eels hiding out in their burrows without regular visitors and set up Facetime visits to try to coax them back into the open!

Conclusions

Divers and snorkelers have fought to capture images – or even glimpses – of spotted garden eels for decades. These shy fish are masters at blending into beds of seagrass. And they’re every bit as enchanting in a saltwater aquarium. But you need to understand the challenge you’re taking on. You’ll be setting up a reasonably sparse tank full of sand. And feeding time? It’s tricky. But if you can work out all of the kinks, it’s worth it to watch these fish pop out of their burrows.

References

• Allen, G., Steene, R., Humann, P., and Deloach, N. 2005. Reef Fish Identification – Tropical Pacific.
• Paxton, J.R. 1998. Encyclopedia of Fishes.
• Randall, J.E., Allen, G.R., Steene, R.C. 1997. Fishes of the Great Barrier Reef and Coral Sea.
• Smith, D.G. 1989. Fishes of the Western North Atlantic.