Four-Eyed Fish

Where they come from (and why they’re so weird)

Four-Eyed Fish (Anableps anableps) come from mangroves and coastal river mouths in northern South America—think muddy, tidal water where fresh and salt mix all day. That habitat explains basically everything about them: they live at the surface, they like current and open space, and they handle shifting salinity way better than most fish.

And yeah, the “four eyes” thing is real. Each eye is split so they can look above and below the waterline at the same time. It’s as cool as it sounds, but it also means your setup has to respect their surface-heavy lifestyle.

Setting up their tank

These aren’t “cute oddball community fish.” They get big, they’re active, and they use the whole top layer like a racetrack. If you cram them into a small tank, they’ll stress, scrape themselves up, and act like little jerks to each other.

• Tank size: I wouldn’t bother under 75 gallons for a small group; bigger is genuinely easier with them.
• Footprint matters more than height. Long and wide beats tall every time.
• Lid: tight, gap-free, and weighted if needed. They jump, and they’re good at it.
• Water: brackish. Most people land around SG ~1.005–1.012, but pick a target and keep it steady.
• Filtration: strong. They’re messy eaters and you’ll be feeding at the surface a lot.
• Flow: moderate with good surface movement. They hang at the top, so oxygenation helps a ton.

Decor-wise, give them open swimming space up top. I like keeping hardscape lower—mangrove-style roots, rocks, or tough plants that tolerate brackish. Just don’t build a surface-level obstacle course; they’ll clip themselves and you’ll see split fins and scraped noses.

What to feed them

They’re surface feeders with a big appetite. Mine acted like tiny piranhas at the top, but they’re not picky once they recognize you as the food source.

• Staples: quality floating pellets or sticks (predator/livebearer-style foods work well).
• Frozen foods: krill, mysis, chopped shrimp, bloodworms (as a treat), and good frozen blends.
• Fresh treats: small bits of seafood like clam or shrimp—rinse it first.
• Occasional greens: spirulina-based foods or blanched veg can help keep things moving.

Behavior and tankmates

They’re social and you’ll get better behavior in a group, but they also establish a pecking order. Expect some chasing, especially at feeding time or if the tank’s cramped.

They’re also constant motion. If you want a calm, zen tank—this isn’t that. They’re more like a busy shoreline scene.

• Keep them in a group (I’ve had the best luck with 5+ in a big tank).
• Aim for tankmates that like brackish water, can handle boisterous surface activity, and aren’t small enough to be harassed or eaten.
• Avoid slow, delicate fish and anything that hangs at the surface and can’t compete at feeding time.

Breeding tips (if you want to go there)

Breeding them is doable, but it’s not like guppies where you blink and there are 50 babies. Four-Eyed Fish are livebearers, and the wild part is the males have “left” or “right” sided anatomy—pairing matters.

• Keep a group so compatible pairs can happen naturally (that left/right thing makes forced pairing frustrating).
• Lots of food and stable brackish conditions help get them in the mood.
• Pregnant females get noticeably fuller, but they’re still active surface fish—don’t expect them to hide much.
• If you want to raise fry, have a separate brackish grow-out ready; adults will eat small fry if they get the chance.

Common problems to watch for

Most issues I’ve seen with Anableps come from three things: not enough space, sloppy brackish mixing, and surface injuries from bad tank design (or a bad lid situation). Fix those and you’re already ahead of the game.

• Jumping and lid injuries: tiny gaps are enough. Check around hoses and cables.
• Scrapes on the back/top: they race the surface and smack decor or the lid, especially during feeding frenzies.
• Fin damage from squabbles: usually a sign the group is too small or the tank is too tight.
• Chronic stress from wrong salinity: they’ll look washed out, clamp fins, or act skittish.
• Poor water quality: they’re hardy in the “tough fish” sense, but high waste will still burn them—especially at the surface where oxygen matters.

If you nail the big tank + steady brackish water + tight lid combo, Four-Eyed Fish are insanely rewarding. They’ll recognize you, swarm the surface at feeding time, and you’ll catch them “staring” above the waterline like they’re plotting something.