Thin sand-eel

Where they come from

Thin sand-eels (Yirrkala tenuis) are one of those oddball marine eels that spend a lot of their time buried, poking just the head out like a periscope. They show up in Indo-Pacific coastal areas where the bottom is sandy or silty, often around rubble zones and gentle slopes.

That habitat tells you almost everything about how to keep them: they want to hide in sand, they hate chaos, and they do not appreciate being forced to live on bare glass with nowhere to disappear.

Setting up their tank

This is an expert fish mostly because of two things: it is a burying specialist and it can be a feeding headache at first. If you get the substrate and the cover right, you are already ahead of the game.

• Tank size: I would not bother under 40-55 gallons for one. Bigger is nicer because you can build a wide sandy footprint and still keep stable water.
• Substrate: fine sand, deep. Aim for 3-5 inches. Sugar-sized aragonite works well. Avoid sharp crushed coral.
• Aquascape: keep rockwork stable and sitting on the glass or on supports, not on top of the sand. Eels burrow and can undermine rocks.
• Flow: moderate. Enough to keep detritus from settling everywhere, but not so much that it turns your sandbed into a dune field.
• Cover: tight lid. Any eel-shaped fish plus an uncovered gap is a bad combo. Seal overflows and cable cutouts.
• Lighting: they do not care. Give them shaded areas and you will see them more often.

I like to give them a couple of "starter burrows" by placing a short length of PVC (like 3/4 to 1 inch) under the sand near a rock edge. They do not always use it, but it seems to help new arrivals settle instead of glass-surfing or vanishing for a week.

What to feed them

Think small meaty marine foods, and think "movement". New thin sand-eels often ignore anything that just sits there. Once they recognize your feeding routine, they usually get a lot easier.

• Good staples: enriched mysis, finely chopped shrimp, chopped clam, chopped squid, krill pieces (sparingly), quality frozen marine blends.
• Live foods to get them started: live ghost shrimp (salt-acclimated), live mysids, blackworms (if you can source them clean), small live fish only as a last resort.
• Enrichment: soak foods in a vitamin + HUFA supplement a couple times a week, especially early on.

Target feeding helps a lot. Use a long feeding stick or turkey baster and place food right in front of the head when it is peeking out. If it is fully buried, do not dump food in and hope. You will just feed the tank and foul the sandbed.

Feeding frequency depends on size and how well it is taking prepared foods. I usually start with small offerings daily, then move to 3-5 times a week once it is steady and not acting frantic at feeding time.

How they behave and who they get along with

Most of the time they are calm, secretive, and honestly pretty entertaining once you learn their schedule. You will get the classic "head out of the sand" pose, little repositioning wiggles, and quick darting grabs at food.

The main compatibility issue is not aggression, it is them getting outcompeted or stressed into hiding. Fast, pushy eaters can keep them from getting food, and big predatory fish can view them as a snack.

• Good tankmates: peaceful to semi-peaceful fish that are not hyper at feeding time (many gobies, blennies, smaller wrasses that are not bullies, reef-safe angels with caution, calm tangs in larger tanks).
• Use caution: dottybacks, big hawkfish, triggers, larger wrasses, and anything that rushes the food like it is a sport.
• Avoid: fish that eat eels, very large groupers, lionfish, big morays, and crustaceans you are attached to (small shrimp can disappear).

With other eel-ish fish, I would not assume peace. Some setups work, but in smaller tanks you are rolling the dice, especially if burrowing spots are limited.

Breeding tips

I have never seen confirmed captive breeding of Yirrkala tenuis in the hobby, and I would not build a plan around it. Like many marine eels, they likely have a larval stage that is not something most of us can rear at home.

If you ever end up with two that tolerate each other, the best you can do is keep the tank stable, keep them well fed, and watch for any seasonal behavior changes. But realistically, this is a "keep it healthy for years" species, not a breeding project.

Common problems to watch for

Most issues trace back to stress, feeding, and substrate. If you nail those, you dodge a lot of headaches.

• Not eating after arrival: very common. Try live foods, dim the lights, reduce competition, and target feed near its burrow.
• Starvation by competition: it is easy to miss because they hide. Watch the body line and make sure it actually grabs food.
• Sandbed rot and nutrient spikes: uneaten meaty food buried in sand gets nasty. Feed small amounts and remove leftovers.
• Rockwork collapses: caused by burrowing under unstable structures. Set rocks on glass and keep heavy pieces secured.
• Jumping/escaping: gaps in lids, overflows, and plumbing cutouts are the usual exit routes.
• External parasites and bacterial issues: stressed new eels can show cloudy skin, frayed fins (if any are visible), or rapid breathing. Quarantine is tricky but still worth the effort with a sand tray approach.

Last thing: keep a flashlight handy for nighttime checks. A lot of sand-eel activity happens after lights out, and seeing it out and moving around at night is often the first sign it is settling in and feeling secure.