Compressed ilisha

Where they come from

Compressed ilisha (Ilisha compressa) is a small coastal clupeid - basically a herring/sardine type fish - that shows up in marine and brackish edges around the Indo-West Pacific. Think bays, estuaries, and nearshore water where the current brings food to them. They are built to cruise and pick plankton out of moving water, not to sit around a rock pile.

Setting up their tank

Give them space and flow first, decorations second. These fish do best in a long tank where they can keep swimming without constantly turning. A short, tall tank usually ends in nose rubs and panic laps.

• Tank size: bigger than you think. I would not bother under 180-240 cm (6-8 ft) length for a small group, and longer is better.
• Group size: they calm down in a group. Aim for 6+ if you can support it, otherwise skip them.
• Flow and oxygen: strong surface agitation, big skimmer, and high dissolved oxygen. These fish act "fine" right up until they are not.
• Aquascape: keep the middle open. Put rockwork tight to the back/sides so they have a clear racetrack.
• Lighting: moderate is fine. What matters more is that they can see food in the water column.

For water, I keep them in full marine salinity and stable temps. They will tolerate some variation in nature, but in a glass box swings just add stress. If you are mixing saltwater, match salinity and temperature during water changes instead of "close enough".

What to feed them

They are water-column feeders. If food is not drifting past them, they often ignore it even if they are hungry. The first goal is getting a strong feeding response. Once they are taking food confidently, everything gets easier.

• Best starters: enriched live or frozen copepods, small mysis, finely chopped krill, cyclops, calanus, and artemia (better enriched than plain).
• Prepared foods: small marine pellets and flakes can work, but introduce them mixed in with frozen so they learn the "shape".
• Feeding style: multiple small feedings beats one big dump. Think 2-4 times a day if you want them to hold weight.
• Technique: feed into the current so it suspends. If it drops straight to the bottom, they miss half of it.

Watch bellies and backs. A fish that is eating but still getting pinched behind the head is not actually getting enough. These guys burn calories just by being themselves.

How they behave and who they get along with

They are nervous, schooling, open-water fish. In a good setup they cruise steadily and snap at food. In a bad setup they ping-pong, hide in corners, or slam the glass.

• Temperament: peaceful. They are not looking for trouble.
• Best tankmates: other calm midwater fish that will not harass them and will not outcompete them at feeding time.
• Avoid: aggressive fish, nippy fish, and anything predatory enough to view them as snacks.
• Feeding competition: fast, bold eaters (many damsels, some wrasses) can starve them out even without aggression.

Breeding tips

Realistically, breeding Compressed ilisha in home aquaria is not a common win. They are schooling, likely broadcast spawners, and cues are tied to seasonal shifts and large-scale water movement. That said, you can at least get them into "spawning condition" by keeping a big group, feeding heavy on planktonic foods, and maintaining pristine oxygen-rich water.

Common problems to watch for

• Not eating after import: the big killer. Dim the lights, give them a group, and offer small drifting foods (copepods/calanus) in the flow.
• Injury from spooking: split fins, nose rubs, missing scales. Fix reflections, reduce sudden movement near the tank, and keep the aquascape from creating dead-end corners.
• Low oxygen events: they breathe hard at the surface or hang in high-flow areas. Increase aeration and surface agitation, clean clogged filters, and watch nighttime O2 dips.
• Wasting away: usually underfeeding or being outcompeted. Add feeding sessions, change food particle size, and separate bully eaters.
• Parasites from wild-caught fish: flashing, excess slime, rapid breathing. Quarantine is worth the effort, but use medications carefully because stressed pelagic fish can crash quickly.

If you are set on keeping them, build the tank around their needs: long swimming room, steady marine water, lots of oxygen, and a feeding routine that keeps food in the water column. Get those right and they go from "impossible" to just plain demanding.