Slender lightfish

Where they come from

Slender lightfish (Vinciguerria attenuata) are open-ocean mesopelagic fish. In the wild they hang out in deep water by day and move up toward the surface at night to pick off tiny drifting critters. That day-night commute (diel vertical migration) is basically the whole story with this fish, and it explains why they are so hard in aquariums.

Setting up their tank

If you are picturing a normal reef tank with bright LEDs and rockwork, flip that idea around. These are pelagic, low-light, nervous fish that do best in a calm, dim, open-water setup. Think of them more like deepwater baitfish than a display fish that will pose for you.

• Tank size: bigger helps more than you think. A long footprint gives them room to cruise without panic turns. I would not try them in anything under 75 gallons, and 125+ is where things get noticeably less stressful for the fish.
• Layout: open water in the middle, with structure only along the back/sides. Use dark rock or even just blacked-out sides to cut reflections.
• Lighting: dim. Blue-heavy is easier on them. Give them a slow ramp and a real dusk period. Sudden lights-on is a recipe for collisions.
• Flow: gentle to moderate, but not blasting. You want suspended food to stay in the water column without pinning the fish to one side.
• Filtration: oversize it. You will be feeding small foods often, and the tank gets nutrient-heavy fast.
• Cover: tight lid. Spooked lightfish will jump.

Stability matters because these fish do not handle extra stress well. Keep salinity steady (around natural seawater, 1.025-1.026), keep oxygen high, and do not let CO2 creep up at night. A skimmer and strong surface agitation are your friends here.

What to feed them

Feeding is where most attempts fail. They are micro-predators that expect a steady trickle of tiny moving prey in the water column, mostly after the lights dim. If you feed like you would for a clownfish, they will slowly fade out even if they look like they are pecking.

• Best starters: live copepods, live enriched Artemia nauplii, small live mysids if you can get them tiny enough.
• Good frozen options (once they are taking food): cyclops, calanus, finely sieved mysis, roe, small krill fragments (sparingly).
• Enrichment: use HUFA enrichment for live Artemia and consider soaking frozen foods. These fish burn energy fast.
• Schedule: multiple small feeds, especially around and after dusk. An auto-feeder is not much help unless you are using tiny pelagic foods; they do not usually take dry.
• Technique: broadcast feed into flow so the food stays suspended. Target feeding at the surface often just makes them bolt.

How they behave and who they get along with

Slender lightfish are shy, schooling-ish, and easily spooked. They will hover and cruise in the water column and spend a lot of time oriented into gentle flow. In a small tank they turn into glass surfers.

• Best kept in a small group if you can get multiple healthy specimens at once. Singles are often more nervous.
• Tankmates should be calm and non-competitive at feeding time.
• Avoid anything that is fast, nippy, or aggressive, and anything that will outcompete them for tiny foods.

Predators are an obvious no (groupers, lionfish, big hawkfish), but the sneaky problem is "peaceful" fish that are just too enthusiastic. If you have to choose, build the tank around the lightfish, not the other way around.

Breeding tips

Realistically, breeding Vinciguerria attenuata in home aquariums is not happening for most of us. They are open-ocean spawners with pelagic eggs/larvae that need planktonic foods on a scale that is hard to provide, plus you would need a setup that supports natural nightly behavior without constant stress.

Common problems to watch for

• Starvation: the number one killer. They need frequent, appropriately sized foods and a calm feeding environment.
• Impact injuries: nose damage, split fins, missing scales from spooking into glass. Dimming lights, blacking out sides, and keeping them in a larger tank helps.
• Shipping stress and rapid decline: many arrive already compromised. Look for fish that are upright, responsive, and not gasping or spiraling.
• Low oxygen/high CO2: they come from well-oxygenated water. Nighttime oxygen dips can hit them hard.
• External parasites: marine ich/velvet can wipe them out quickly, and stressed fish show symptoms fast.

If you take one lesson from folks who have kept them alive the longest, it is this: reduce stress, feed tiny foods often, and keep the tank calm and dim. Every shortcut you take shows up fast with this species.