Gracile rivulus (Kryptolebias)

Where they come from

Gracile rivulus (Kryptolebias gracilis) is one of those little, easy-to-overlook fish that gets really interesting once you dig in. They come from tropical parts of Central/South America where the water can be shallow, plant-choked, and not always "nice" aquarium water. Think slow ditches, puddly edges, swampy backwaters, leaf litter, that sort of vibe.

That background explains a lot: they are tough in some ways, picky in others, and they do best when the tank feels like a messy, cover-filled margin rather than a bright open display.

Setting up their tank

If you try to keep these in a bare tank, you will mostly see a stressed fish hiding and refusing food. Give them structure. Lots of it. They relax fast once they have overhead cover and little "lanes" to move through.

• Tank size: 10 gallons works for a single or a pair, but more floor space makes life easier (especially if you try multiple fish).
• Lid: a tight lid is non-negotiable. They can jump, and they are good at finding gaps around airline tubing.
• Layout: leaf litter, moss, wood, and dense plants. Floating plants help a ton with security and lighting.
• Filtration: gentle sponge filter or a small HOB baffled down. They do not need a river current.
• Substrate: sand or fine gravel is fine. I like dark substrate because it calms them down and you see their colors better.
• Lighting: keep it on the dim side, or at least break it up with floaters.

Water-wise, stable beats chasing numbers. Freshwater is the right call here. Slightly acidic to neutral is usually where they settle in easiest, but the bigger deal is clean water and not letting the tank swing all over the place. They do not love strong flow, but they also do not love stale, dirty water.

What to feed them

These are little predators. In my tanks they have always done best on meaty foods, and they can be annoyingly unimpressed by plain flakes.

• Staples: live or frozen bloodworms, blackworms, daphnia, cyclops, brine shrimp (adult or baby depending on fish size).
• If you can: mosquito larvae are like a cheat code for getting shy fish to eat.
• Pellets: some individuals learn small sinking carnivore pellets, but I would not count on it at first.
• Variety: rotate foods. They do better (and color up nicer) when you do not feed the same thing every day.

Go easy on quantity. They are enthusiastic eaters once settled, but in small tanks it is easy to foul the water. Small portions, more often, beats dumping in a big cube of frozen and hoping for the best.

How they behave and who they get along with

Gracile rivulus are not "community fish" in the classic sense. They are more like little ambush hunters that want personal space. You will see a lot of sitting still, slow stalking, and quick darts at food.

Temperament depends on the individual, but plan for territorial behavior, especially in tight quarters. If you cram them together, you will get fin nips and constant stress.

• Best kept: singly, or a carefully watched pair in a heavily planted tank.
• Tankmates: I usually skip them. If you insist, pick calm, non-nippy fish that will not outcompete them for food.
• Avoid: fast feeders (danios, many barbs), fin nippers, aggressive cichlids, and anything tiny enough to be viewed as food (shrimplets, small fry).
• Inverts: snails are generally fine. Shrimp are a gamble depending on size and hiding spots.

Breeding tips

Kryptolebias are famous because some species can self-fertilize (they are the weirdos of the killifish world), and depending on the exact fish you have, you may see lone individuals producing eggs. That said, do not assume every fish will do that reliably, and do not be shocked if you need a pair to get consistent results.

If you want to try breeding them, set up a simple breeding tank: lots of cover, low flow, and a place for eggs to end up where the adults are less likely to find them.

• Spawning sites: fine-leaved plants (Java moss, guppy grass), mops, or dense roots of floating plants.
• Egg collection: check the moss/mop every day or two and move eggs to a small container with tank water.
• Incubation: keep the eggs in clean water with gentle aeration. Fungus is the main enemy.
• First foods: infusoria/microfoods for the tiniest fry, then baby brine shrimp once they can take it.

Common problems to watch for

Most of the trouble with this species comes from three things: stress from an exposed tank, food issues, and water quality sliding in a small setup.

• Jumping: the #1 heartbreak. Tight lid, no gaps.
• Not eating: usually new fish stress, too much light, too much activity around the tank, or food that is too dry/boring. Start with live/frozen.
• Skinny fish: can be internal parasites, or just being outcompeted if housed with faster tankmates.
• Fungus on eggs: common if you are collecting eggs. Keep water clean, add gentle aeration, and remove bad eggs fast.
• Fin damage: usually from cramped quarters or incompatible tankmates. Add cover or separate fish.
• Ammonia/nitrite spikes: small tanks + heavy foods = trouble. Feed smaller and do more frequent water changes.

If you give them a calm, covered tank and feed them like the little predators they are, they are genuinely rewarding. They are not a "show fish" that parades around, but once they trust the setup, you will catch a lot of cool behavior in the plants and leaf litter.