Small lanternfish

Where they come from

Small lanternfish like Diaphus roei are mesopelagic fish - open-ocean, midwater drifters that spend their days deep and their nights higher up in the water column. They are part of the whole "deep scattering layer" thing you read about in ocean docs. In the wild they live under dim light, steady conditions, and a constant rain of tiny drifting food.

Setting up their tank

Think "quiet pelagic holding system" more than "display reef." You want stability, oxygen, and gentle, uninterrupted swimming space. They spook easily and they do not appreciate bright lights or chaotic flow.

• Tank size: bigger is better because it spreads out aggression and reduces panic. I would not try them in anything under 75 gallons, and 120+ is a lot easier.
• Shape: long tanks work better than tall cubes. Give them a clear lane to cruise.
• Lighting: dim. If you want to see them, ramp up slowly and keep a lot of shaded areas. Red/low-spectrum viewing light helps.
• Flow: gentle and even. Avoid blasting powerheads that pin them to a corner. Diffuse returns, use gyres on low, or big sponge guards.
• Filtration: oversized skimming and lots of bio. You will be feeding small foods often and water gets dirty fast.
• Cover: tight lid. Lanternfish can jump when startled. Tiny gaps still count.
• Acclimation: slow and calm. Keep the room lights down and do not chase them with a net. I use a container and gently pour them through water exchanges instead.

Water quality is a balancing act. You need enough feeding to keep weight on them, but that same feeding will wreck your nitrate and oxygen if your system is small or under-filtered. I like heavy aeration at night and a skimmer that pulls dark, nasty stuff daily.

What to feed them

This is where most attempts fail. Diaphus are built to pick off tiny drifting prey, not to smash big chunks. A lanternfish that is eating is usually a lanternfish that lives. A lanternfish that is not eating for a week is usually on a timer.

• Best starting foods: live copepods, enriched adult brine (not as a staple, but great to kick-start feeding), live mysis if they will take it.
• Staple foods once settled: frozen calanus, cyclops, small mysis, finely chopped krill/clam (only if they can actually swallow it), enriched Artemia, roe-type foods.
• Enrichment: HUFA/omega enrichments for brine and mysis. They burn calories fast.
• Feeding schedule: small amounts 3-6 times per day beats one big dump. If you can do an auto-feeder with tiny pellets, great, but do not assume they will take dry food.
• How to get them started: feed in low light, turn down flow for 15-20 minutes, and use a feeding ring or a gentle "food cloud" in midwater.

Watch the belly profile from the side. You want them slightly rounded after a meal, not pinched. If you see a sharp "knife belly" or a hollow behind the head, increase feeding frequency and switch to higher-calorie items like calanus and enriched mysis.

How they behave and who they get along with

They are shy, midwater fish. Mine spent the day hovering in the dim and then got more active as the lights went down. They do not like being the center of attention, and they stress out if bigger fish rush the food.

• Best tankmates: calm, non-competitive fish that will not outcompete them for tiny foods. Think small, peaceful planktivores and very mellow community fish.
• Avoid: aggressive feeders (wrasses that blitz food, most anthias in a frenzy, dottybacks), anything that might see them as a snack, and nippy fish that harass midwater swimmers.
• Grouping: they often do better in a small group if you can source them together and your tank is large enough. Singles can sulk. A group can be bolder, but only if food is abundant.

They spook. Sudden light changes, banging doors, hands in the tank, a big fish charging past - all of that can send them into a panic sprint. Give them hiding shade (not caves they can wedge into, more like dark overhangs and tall rock shadows) and keep the tank in a low-traffic area if you can.

Breeding tips

Realistically, breeding Diaphus roei in home aquariums is not something hobbyists are reliably doing. They are open-ocean spawners with pelagic eggs and larvae that need specialized planktonic rearing. Even public aquariums rarely report success with lanternfish breeding.

Common problems to watch for

• Starvation: the most common. They look "fine" until they suddenly do not. Track body condition weekly and adjust feeding before they get thin.
• Collection and decompression damage: deepwater fish can arrive with buoyancy issues, internal trauma, or just "off" behavior. Buy only from sources that handle deepwater species carefully.
• Light stress: bright reef lighting can keep them pinned in a corner and refusing food. Dim zones help a lot.
• Jumping and impact injuries: tight lid and calm tank. Spooked lanternfish can launch into covers or overflows.
• Oxygen dips: heavy feeding plus warm water can drop O2, especially at night. Add aeration and keep surface agitation up without making the flow chaotic.
• Parasites and bacterial issues after shipping: look for rapid breathing, flashing, excess slime, frayed fins. Quarantine is worth the effort if you can provide low light and frequent feeding in QT too.

If you are set on trying them, plan the system around them, not around your corals. Dim, calm, clean, and fed often. That is the formula that finally made them "click" for me.