Northern smoothtongue

Where they come from

Northern smoothtongue (Leuroglossus schmidti) is a deepwater, cold-sea smelt relative from the North Pacific and nearby subarctic seas. Think Aleutians/Bering Sea vibes: dark water, steady cold temps, and a life spent hovering and snapping up tiny drifting prey.

Setting up their tank

If you are serious about keeping this species, start with the chiller plan first and build everything else around it. Warm marine tanks, even "cool room" temps, are a slow death for deepwater cold species.

• Temperature: cold. You are generally looking at 39-50F (4-10C). Pick a target and hold it steady.
• Tank size: bigger than you think for a "small" fish. A long footprint helps because they cruise and startle easily. I would treat 75+ gallons as a practical starting point, larger if you want a group.
• Flow and oxygen: high dissolved oxygen is your friend. Use strong surface agitation and oversized filtration. Cold water holds more oxygen, but these fish still like it well-aerated.
• Lighting: keep it dim. Bright reef lighting stresses them out. Use shaded areas and low-intensity LEDs.
• Aquascape: open water with a few dark hide zones. I like minimal rockwork and no sharp decor. They spook, bolt, and can scrape themselves.
• Lid: tight-fitting. Spooking equals jumping.

For substrate, I keep it simple: bare bottom or fine sand. Bare bottom makes it easier to keep food from rotting, and rotting food is a big deal in a cold tank because you usually run lower overall turnover than a reef.

What to feed them

They are small-predator planktivore types. In captivity the main battle is getting consistent feeding without fouling the water. They often arrive skinny, and they do not always recognize pellets right away.

• Best starters: live or freshly hatched foods (enriched live brine, copepods, mysis) to trigger feeding.
• Frozen staples once they settle: PE mysis, chopped krill, finely chopped shrimp, calanus/copepod blends.
• Training foods: small sinking marine pellets can work, but I usually transition slowly by mixing pellets into thawed mysis.
• Feeding schedule: small portions 1-2 times daily beats one big dump.

Watch bellies, not just "I saw it bite." A shy fish will dart at food and miss. If you keep more than one, make sure each one is actually putting on some weight over a couple weeks.

How they behave and who they get along with

Northern smoothtongues are more spooky than aggressive. They hang in the water column, then do quick snaps at passing prey. The big personality trait is how easily they stress from commotion, bright light, and pushy tankmates.

• Temperament: generally peaceful, but will eat anything that fits in the mouth.
• Group vs single: a small group can work if the tank is long and calm, but shipping stress makes adding multiples at once risky. Quarantine and add carefully.
• Best tankmates: other coldwater species that are calm and not food-competitive (think slow, non-nippy fish).
• Avoid: aggressive feeders, fin nippers, and anything tropical. Also avoid crabs that might grab them when they rest.

Breeding tips

Realistically, breeding this species in home aquaria is a long shot. They are deepwater fish with seasonal cues (temperature shifts, day length, and probably pressure/vertical migration signals) that are hard to replicate.

If you still want to experiment, the best you can do is focus on long-term conditioning: keep them cold and stable, feed a varied diet heavy on small crustaceans, and run a seasonal light cycle with a gentle winter cool-down and spring warm-up (still within coldwater range). Spawning, if it happens, will likely be broadcast eggs that drift and get eaten or filtered out.

Common problems to watch for

• Heat stress: rapid breathing, hanging at the surface, refusing food. This is the big one.
• Shipping/decompression trauma: odd buoyancy, rolling, inability to hold position. Sometimes they never recover.
• Starvation after import: they may "pick" without swallowing much. Track body condition weekly.
• Injury from startle-jumping: scraped snout, torn fins, sudden death from carpet surfing if the lid has gaps.
• Ammonia spikes from uneaten food: cold tanks can lull you into overfeeding because the fish move slowly. Test and siphon leftovers.
• Parasites and bacterial issues: stressed imports can show cloudy eyes, frayed fins, or skin haze. Quarantine is your best tool.

If I had to sum it up: stability, cold water, low stress, and disciplined feeding. Get those right and you have a shot. Cut corners on any one of them and this fish will remind you why the difficulty label says expert.