Flagtail

Where they come from

Kuhlia petiti, the marine flagtail, is a fast, schooling fish from the tropical Pacific. You see them around reef passes, surge channels, and lagoon inlets where the water really moves. Juveniles wander into brackish estuaries, then head back to coastal marine water as they grow. Think bright, silvery sprinters that like to hang in the current.

Setting up their tank

These guys want length, flow, and clean, well-oxygenated water. They are bulletproof jumpers and they never stop moving, so plan a tank around open swimming lanes and a rock layout that keeps the front half of the tank clear.

• Size: 180+ gallons for a group in a 6 ft (or longer) tank. A single fish can work in 120 gallons, but it's a compromise.
• Salinity: 1.023-1.026 for full marine. If your fish came from brackish, step up salinity about 0.002-0.003 SG per week.
• Temperature: 76-80 F (24-27 C).
• pH/alk: Reef range (roughly pH 8.0-8.4) with stable alkalinity. They hate swings.
• Flow: Strong. Aim for 10-20x turnover with a steady current lane down the length of the tank.
• Aquascape: Rockwork along the back and sides, open water in the middle. Overhangs give them a place to rest between bursts.
• Filtration: Oversized skimmer and solid mechanical filtration. They eat well and make a mess.
• Lid: Tight-fitting with every gap covered, including around cords and overflows.
• Lighting: Anything from moderate to bright works. They do not care much about corals unless food lands on them.

What to feed them

Think active midwater predator with a side interest in planty bits. New arrivals often wake up for moving foods first, then take dry once they settle.

• Staples: PE mysis, chopped shrimp, small krill, finely chopped fish, quality marine pellets that sink slowly.
• Extras: Enriched brine (as a training food), calanus, copepod concentrates, spirulina pellets, tiny nori shards mixed into the feed.
• Frequency: Juveniles 2-3 small meals daily; adults 1-2 decent feeds. They burn calories fast.
• Vitamins: Soak foods in a vitamin/omega supplement 2-3x per week to support color and immunity.

If a new fish refuses prepared foods, start with thawed mysis squirted into the current and mix in pellets over a week. Scatter food across the flow so one bold fish does not hog it all.

How they behave and who they get along with

Flagtails are quick, alert, and social. A proper group is 5+ so any bossy behavior gets spread out. In pairs or trios, the weakest fish often gets ridden hard.

• Good tankmates: Robust, fast swimmers like tangs, larger wrasses, larger anthias, rabbitfish, and bigger damsels. Peaceful groupers that ignore them also work.
• Use caution: Ornamental shrimp and tiny fish can be viewed as snacks, especially once the flagtails hit adult size.
• Avoid: Mean triggers and big, territorial damsels in tight quarters. Flaring dottybacks and slow feeders will get outcompeted at mealtime.

Reef-wise, they are generally fine with corals. They are not polyp pickers. The only time I have seen trouble is when food rests on fleshy LPS and a hungry flagtail goes after the food and bumps the coral.

Breeding tips

Realistically, this is not happening in home aquariums. They are pelagic spawners that use open water and likely time their spawning with tides and moons. Public aquariums with huge lagoon systems might have a shot, but I have not seen a verified home report.

Common problems to watch for

• Jumping and head injuries: Cover the tank. Add a dim ramp-up and ramp-down on lights; sudden blackouts spook them.
• Salinity shock: Many are collected or held in lower salinity. Match what they were kept in, then move to full marine slowly.
• Ich and velvet: Fast fish can still get hammered by parasites after shipping stress. Quarantine new fish and keep copper or alternative treatments ready.
• Refusing food: Start with moving, meaty foods in the current. Add garlic/an attractant sparingly and blend in pellets over time.
• Nutrient creep: They eat like athletes. Skim wet, rinse mechanical filters often, and keep up on water changes.
• Bullying within small groups: Avoid keeping two or three. Either one fish or a proper group of five or more in a big tank.