Longtail pencilsmelt

Where they come from

Longtail pencilsmelts (Nansenia longicauda) are open-water, deep-ish coastal fish from the Southeast Pacific - you see them referenced around the Humboldt Current region (Chile/Peru side). They are built for cruising in cool, oxygen-rich water and picking off tiny drifting prey.

Setting up their tank

This is an expert fish mostly because of environment and feeding, not because it is aggressive or delicate in a dramatic way. They do best in a long tank with open swimming room. Think "racetrack" more than "rock pile".

• Tank size: I would not bother under 75 gallons, and 125+ is way less stressful for them (length matters more than height).
• Aquascape: keep rockwork minimal and low. Leave a big, clear lane across the front and middle for cruising.
• Flow: strong, messy flow. You want water moving everywhere, not a single jet blasting them.
• Oxygen: oversized skimmer and good surface agitation. These fish act "off" fast in stale water.
• Lighting: they are happier under moderate lighting with some shaded zones. Blazing reef lights can keep them jumpy.
• Cover: tight lid. They are jumpers, especially the first couple weeks and during night spooks.

Temperature is the make-or-break for a lot of people. They are much more comfortable on the cool side for marine fish. If your fish room sits at 78-80F year round, plan on a chiller or pick another species.

Salinity and basic parameters are standard marine, but stability matters because they are always burning energy. I also keep nitrate reasonable, not because they melt at 10-20 ppm, but because lower nutrients usually tracks with higher oxygen and cleaner water.

What to feed them

Feeding is the other reason they land in "expert." They are micro-predators and a lot of them come in skinny and stressed. You need to be ready with the right foods and a plan to get weight on them without polluting the tank.

• Best starters: live enriched brine, live copepods, live adult Artemia, small live mysids if you can source them.
• Great frozen once they are taking food: frozen cyclops, calanus, finely chopped mysis, roe, and small krill shards (not big chunks).
• Prepared: some will learn small marine pellets, but I would not count on it early. Use pellets as a long-term convenience, not the main plan.

I have the most success feeding small amounts multiple times a day. They are built to grab frequent tiny meals, not to gorge once. If you dump a big cube of food, half of it blows into a corner, and you get ammonia while the fish still look hungry.

How they behave and who they get along with

They are schooling, midwater cruisers. A single fish usually stays nervous and skittish. In a group, they settle in and you see the natural pacing and quick little snaps at food.

• Group size: more is better. Aim for 6+ if your tank can handle it.
• Temperament: peaceful, but they are easily bullied off food.
• Best tankmates: other cool-water, non-competitive fish that do not harass midwater swimmers.
• Avoid: fast, pushy feeders (many damsels, some wrasses) and anything big enough to treat them as snacks.

They can coexist with inverts that like similar temps, but remember the food you are providing (pods, tiny shrimp) can turn your tank into an expensive buffet for other livestock too. If you keep them with very efficient pod hunters, you will be fighting an uphill battle.

Breeding tips

Breeding in home aquariums is not common. They are pelagic spawners in nature, and the whole cool-water seasonal cue thing is hard to replicate. If you ever get courtship behavior, it is usually tied to a gradual temp shift and heavy feeding.

If you want to take a swing at it, think like a marine baitfish breeder: big group, very stable water quality, and a plan for tiny live foods if larvae appear. Even then, raising the young would be the real challenge.

Common problems to watch for

• Wasting away: the classic issue. They look fine one week, then the belly pinches in. This is usually not enough food frequency, wrong particle size, or tankmates outcompeting them.
• Jumping: stress + spooks. Tight lid, cover overflow gaps, and keep the room dark the first nights after introduction.
• Low oxygen stress: fast breathing, hanging at the surface, listless pacing. Fix flow and aeration before reaching for meds.
• Shipping damage and parasites: many arrive rough. Quarantine helps, but they can be sensitive to heavy-handed treatments in low-oxygen QT setups.
• Food-driven water quality swings: multiple daily feedings can foul water fast if your filtration is sized like a normal reef.

If you keep them cool, give them swimming room, and commit to the feeding routine, they are really rewarding. A settled group looks like a little slice of open ocean in your living room.