Yellowmouth jawfish

Where they come from

Yellowmouth jawfish (Opistognathus nothus) are little burrow-dwelling weirdos from the tropical western Atlantic and Caribbean area. Picture sandy slopes and rubble zones where they can dig in and hover above the entrance like a tiny guard dog. That whole lifestyle drives basically every success or failure you will have with them.

Setting up their tank

If you treat this like a normal reef fish, you will be frustrated. A yellowmouth jawfish wants a burrow first, and everything else second. Give it the right bottom, and a lot of other "mystery problems" just disappear.

Tank size is less about gallons and more about footprint and peaceful space. A 20 long can work for a single fish if you build the bottom right and keep tankmates mellow. Bigger makes life easier because you can give it a quiet corner away from traffic and aggressive feeders.

• Substrate depth: aim for 3-5 inches if you can, with a mix of fine sand and small rubble bits
• Rubble material: small shells, tiny coral skeleton pieces, pea-sized rock chips (they love "building supplies")
• Rockwork: stable and sitting on the glass or a solid base, not perched on sand they might excavate
• Flow: moderate overall, but try to keep the burrow zone from getting blasted
• Lighting: they do not care much, but avoid putting the burrow where your brightest lights cook the sandbed

Stability matters more than chasing a magic number. Keep salinity steady (jawfish hate swings), keep nitrate reasonable, and do not let oxygen dip at night. They sit in a hole and breathe what is around them, so stale, low-oxygen corners can hit them harder than a fish that cruises in the open water.

What to feed them

They are planktivores with attitude. In the wild they grab passing meaty bits, and in a tank they do best when you feed small foods more often rather than one big dump. Mine always ate better once they felt secure in a burrow and could pop out to snatch food, then retreat.

• Frozen: mysis (smaller pieces), brine shrimp with enrichment, finely chopped krill, calanus, plankton blends
• Live (great for new or picky fish): live brine, copepods, blackworms (rinse well, use sparingly)
• Prepared: small sinking pellets or soft pellets once they recognize them (train with frozen first)

Watch the tank during feeding. Jawfish are not built to out-compete trigger-happy wrasses or anthias in a food frenzy. If you see it staying down while everyone else eats, you either need to change the feeding approach or rethink the tankmates.

How they behave and who they get along with

A comfortable jawfish is one of the most fun fish you can keep. It will hover, rearrange rubble like a contractor, and keep an eye on you from its doorway. A stressed jawfish is a ghost that hides, refuses food, or launches itself through a lid gap.

They are generally peaceful, but they get very serious about their personal real estate. They also do not love boisterous fish that buzz the sandbed or constantly investigate the burrow.

• Good tankmates: small gobies, blennies that stick to rock, peaceful clowns, banggai cardinals, small fairy/flasher wrasses (with caution)
• Questionable: larger wrasses, dottybacks, hawkfish, aggressive clowns, sand-sifting gobies that remodel the bottom nonstop
• Avoid: triggers, puffers, large angels, big hawkfish, anything that picks at the burrow or bullies at feeding time

If you want more than one, plan the whole bottom around it. Give them space and multiple rubble piles so each can build. Even then, expect squabbles. Most hobbyists are happiest with a single specimen unless they have a larger, calmer setup.

Breeding tips

Jawfish breeding is cool because you can actually see the behavior. The male mouthbroods the eggs. In a settled pair, you may notice courtship hovering and the male staying close to the burrow entrance with a "full mouth" look.

• If you want a shot at breeding: keep them in a species-focused tank with low competition at feeding time
• Feed heavy on small meaty foods to condition them
• Keep the burrow zone calm and stable so the male does not spit the clutch from stress
• If larvae are released: be ready with a larval trap or gentle overflow collection, plus live foods on day one

Common problems to watch for

Most yellowmouth jawfish losses trace back to three things: jumping, stress from not having a real burrow setup, and getting outcompeted or harassed. If you fix those, your odds go way up.

• Jumping through tiny gaps - lid every opening, including around cables and overflow teeth
• Refusing food after introduction - often a sign it does not feel secure; add rubble, reduce traffic, target feed
• Collapsed burrows - substrate too shallow or too uniform; mix grain sizes and provide building material
• Rockwork shifts after digging - always place rocks on a stable base, not on sand
• Bullying at the burrow - watch for fish that hover over the entrance or dive-bomb it
• Shipping stress and disease susceptibility - quarantine if you can, and avoid rapid salinity changes

One last real-world tip: give them time. A new jawfish can act "broken" for a week or two, then suddenly decide the burrow is home and flip into full personality mode. If it has a secure hole, a calm neighborhood, and food drifting past the door, you are usually on the right track.