Cricare killifish

Where they come from

Xenurolebias cricarensis (the Cricare killifish) is one of those South American annual killis that feels like it was designed to humble experienced fishkeepers. They come from temporary pools around the Cricare region in Brazil - water that shows up for a season, then disappears. The fish live fast, spawn hard, and the next generation waits out the dry spell as eggs in the substrate.

Setting up their tank

Keep the tank simple and built around stability. I have had the best luck with a small species tank where you control everything: light, temperature, water chemistry, and especially the substrate they will use for spawning.

A 10-15 gallon works for a trio or a small group if you keep up with maintenance. They do not need a tall tank. They do appreciate floor space and cover so they do not feel exposed.

• Tank size: 10-20 gallons is a sweet spot for managing water quality without swinging parameters too fast
• Filtration: gentle sponge filter (air-driven) so fry and adults do not get blasted
• Flow: low - they come from still, shallow water
• Cover: floating plants and clumps of moss or fine-leaved stems to break lines of sight
• Lid: tight fitting - killis jump, and annuals seem to do it at the worst possible time

For water, aim soft and on the acidic side. You do not need lab-grade numbers, but you do want consistency and low hardness. If your tap is hard, mix in RO/DI and remineralize lightly. I keep them warmer than most people expect, but not hot - mid 70s F is a comfortable place to start and tweak from there based on the strain and your room.

The big decision is spawning substrate. These are peat spawners. You can run a bare-bottom tank with a removable peat container (my preference), or you can use a thin peat layer in a tray. Removable is cleaner and makes egg collection way easier.

• Spawning setup I use: a plastic food container with a hole in the lid, filled with boiled/rinsed peat or coco peat
• Depth: a couple inches of peat in the container so they can really dive in
• Placement: where you can still siphon around it and keep waste from building up

What to feed them

Live and frozen foods are your bread and butter here. Most Cricare killifish I have kept ignored dry foods or only picked at them. They color up, spawn, and stay in better shape on meaty stuff.

• Staples: frozen bloodworms, blackworms (if you can get clean ones), daphnia
• Great conditioning foods: live baby brine shrimp for younger fish, grindal worms, mosquito larvae (collected safely)
• Occasional: high-quality micro pellets if they will take them, but do not rely on it

Feed small amounts more often rather than one huge dump. They are pigs, and heavy feeding plus warm water can foul a small tank quickly. I usually do 1-2 feedings a day, and I vacuum the bottom lightly every couple days.

How they behave and who they get along with

They are not community fish in the way people mean it. Males can be pushy, especially in tight quarters, and females get harassed if you do not give them cover and enough space. In a species tank with plants and a peat box, their behavior is way more relaxed.

Tankmates are usually a bad trade. Small tetras and similar fish can outcompete them for food, stress them, or snack on eggs and fry. Shrimp are hit-or-miss - adults may ignore big shrimp but will absolutely eat babies.

• Best option: species-only tank
• If you must mix: pick calm, tiny fish that like the same soft acidic water, and accept you may not get eggs/fry
• Keep 1 male with 2-3 females if you want less constant sparring

Breeding tips

Breeding is the main reason people keep Xenurolebias, and also the part that makes them "expert" level. You are basically managing a fish and an egg bank at the same time. Adults spawn into peat, the eggs need a dry incubation phase, and then you "wet" them to hatch.

Condition the adults well, then give them a peat container. They will dive in and deposit eggs over days to weeks. I like to leave the container in for 1-2 weeks, then swap it for a fresh one so I can stagger batches. That way you are not betting everything on one incubation.

• Collecting peat: squeeze gently until it is damp, not dripping
• Bagging: put peat in a zip bag with a little air, label it with date and species
• Storage: dark place at stable room temp (avoid baking hot shelves)
• Checking: every couple weeks, open and sniff - sour smell usually means too wet and going anaerobic

Incubation length varies with species, line, and temperature. With annuals, patience is part of the game. If you wet too early you get nothing or weak hatches. If you wait too long you can miss the best hatch window. I usually test-wet a small pinch first rather than the whole bag.

For hatching, use shallow water in a small container, same soft water you keep the adults in. Some people add a tiny bit of infusoria starter or a leaf for microorganisms. Once fry are up, they need tiny live foods quickly. Baby brine shrimp works once they are big enough, but be ready with microworms, vinegar eels, or infusoria for the first days if the fry are small.

• First foods: infusoria (or a mature sponge filter squeeze), vinegar eels, microworms
• After they can take it: baby brine shrimp
• Grow-out: frequent small water changes and lots of food, but keep it clean

Common problems to watch for

Most issues come from water quality swings, overfeeding, or rushing the incubation process. These fish do not give you much time to "fix it later." If something is off, they show it fast: clamped fins, hiding, refusing food, or sudden losses.

• Bloating and stringy poop: often from rich foods or internal issues - switch to lighter foods like daphnia and tighten up water changes
• Mouth fungus/fin rot: usually follows stress or dirty water - clean up the tank and treat early
• Jumping: gaps in lids and airline holes are a real problem
• Egg loss in peat: too wet (anaerobic), too dry (desiccation), or overheated storage

If you are struggling, simplify. Species tank, sponge filter, soft water, heavy cover, controlled feeding, and a removable peat container. Once you can keep adults steady for a couple months, the breeding side gets much less mysterious.