Paracatu rivulus (killifish)

Where they come from

Paracatu rivulus (Melanorivulus paracatuensis) is one of those little South American rivulus killifish that shows up from seasonal creeks and shallow margins around the Paracatu drainage in Brazil. Think warm, plant-choked edges, leaf litter, slow water, and a lot of hiding spots. They are not a "big river" fish - they are built for puddles, ditches, and quiet backwaters.

Setting up their tank

If you set these up like a mini nature scene instead of a clean display tank, they settle in fast. They are small, but they are active and curious, and they appreciate cover more than open swimming space.

• Tank size: 10 gallons works for a pair or trio (1 male, 2 females). Bigger is easier if you want a group.
• Lid: non-negotiable. Rivulus are jumpers, and they will find the one gap around a filter cord.
• Filtration: gentle sponge filter or a baffled HOB. They do not love being blasted around.
• Substrate: sand or fine gravel. Leaf litter on top looks great and they use it.
• Hardscape: driftwood twigs, piles of leaves, and plant thickets. Give them sight breaks.
• Plants: floaters (Salvinia, frogbit), and dense stuff like Java moss, guppy grass, or crypts.
• Lighting: moderate. Floaters help them feel safe and bring out color.

Water-wise, they are pretty adaptable as long as it is clean and stable. I have kept rivulus types in neutral to slightly acidic water without drama. Aim for warm-but-not-hot temps and do small, regular water changes rather than big swings.

What to feed them

These are micropredators. They can learn dry foods, but they really come alive on small live and frozen stuff. If you want color and breeding, food variety is your friend.

• Staples that work great: frozen baby brine shrimp, daphnia, cyclops, chopped bloodworms (sparingly).
• Live foods (best results): baby brine shrimp, grindal worms, microworms, small daphnia.
• Dry foods: small high-protein pellets or crushed flakes - use as backup, not the whole diet.
• Feeding rhythm: small meals 1-2x daily. They are enthusiastic eaters, but their stomachs are tiny.

How they behave and who they get along with

Expect a bold little fish with a bit of attitude, especially males. They posture, flare, and do short chases, but in a planted tank it usually stays at "showing off" levels.

They are not a great pick for a busy community tank. Fast fish steal their food, and boisterous tankmates keep them stressed and hiding. A species tank is easiest, but you can do tankmates if you choose carefully.

• Good options: small, calm fish that do not outcompete them (chili rasboras, ember tetras), small pencilfish, gentle bottom dwellers like pygmy Corydoras.
• Things I avoid: fin nippers, hyperactive feeders (danios), bigger tetras, most barbs, and anything that treats them like snacks.
• Shrimp: depends. Adults may be fine, but expect baby shrimp to disappear.

Breeding tips

If you have kept other rivulus, the playbook feels familiar. They are not annual killifish, so you are not drying eggs. They scatter eggs in fine plants and moss and will happily snack on them if given the chance.

• Set up: a pair or trio in a small breeding tank with a sponge filter.
• Spawning media: Java moss, spawning mops, or a dense clump of fine-leaf plants.
• Egg collection: check the moss/mop daily and pull eggs to a small container, or move adults out after a week.
• Incubation: keep eggs in clean water with a tiny bit of airflow or gentle water movement. Remove any fuzzy eggs.
• First foods: infusoria or vinegar eels for the first days if fry are tiny, then baby brine shrimp as soon as they can take it.

Common problems to watch for

Most issues I have seen with this group come from three things: jumping, stress from exposure, and slow starvation in community tanks where they cannot compete.

• Jumping: lid every time, and block gaps around hoses and wires.
• Not eating: often a new-tank stress thing or they do not recognize dry food. Try live/frozen and add cover.
• Shy, washed-out colors: tank too bright/bare, or too much traffic from tankmates.
• Ich and other parasites: usually after a temperature swing or new fish. Quarantine new arrivals if you can.
• Fin damage: male sparring in cramped tanks or nippy tankmates. Add plants or separate.