I recently received another batch of annual killifish eggs — this time Xenurolebias pataxo.

These eggs were collected on the 16th of April, and the incubation period for this species is roughly 2–3 months. So technically, I shouldn’t even have been looking at them for another four weeks.

But here’s where experience (and curiosity) kicks in.

When Eggs Don’t Follow the Rules

While checking the outside of the bag, I noticed a couple of eggs stuck to the plastic that looked like they had developed early.

Now, when you’re collecting peat from a breeding tank, you can accidentally mix in older eggs that have been sitting buried for weeks — even months. So sometimes a “fresh” batch isn’t entirely fresh.

Normally, I wouldn’t interfere. But these looked advanced enough that I had to investigate.

This is an important lesson:

Always check stored annual eggs weekly.
Make sure they’re developing properly and that the peat stays slightly moist.

If I open a bag and decide not to wet the eggs, I’ll add just a single drop of water back into the peat to maintain moisture before resealing.

Early Development… But Not Enough

After checking about half the bag, I realised only one or two eggs had developed significantly. The rest were progressing normally.

Wetting the entire batch for just a couple of early eggs would have been risky — you can wipe out the whole lot if they’re not fully ready.

So I re-bagged them and put them back into storage for another four weeks.

The Surprise – Eggs Turned Black

A few days later, I noticed something interesting.

The eggs had gone from a light brown colour to very dark — almost black.

I couldn’t clearly see eye spots, but the colour change was dramatic. I didn’t want to risk them deteriorating in the peat, so I trusted my instincts and wetted them.

They hatched within three minutes.

That’s one of the fastest responses I’ve seen from an annual.

So with this species, don’t rely purely on seeing eye spots. If they suddenly darken significantly, they’re likely ready.

Fry Size – The Opposite of What I Expected

What shocked me most was this:

The eggs are small… but the fry are large.

And not just slightly larger — they were taking newly hatched brine shrimp within hours of hatching.

That makes raising them much easier compared to species that require days of infusoria and paramecium first.

After three days, I moved the fry into fresh water and re-dried the peat to check for any staggered hatchers.

Species Information – Xenurolebias pataxo

This species was previously classified under a different genus before being placed in Xenurolebias.

It’s found inland on the east coast of Brazil, in areas that were once forest and scrubland. Unfortunately, these habitats are being cleared rapidly for housing and agriculture.

I’ve visited the region myself, and the habitat destruction is happening at an alarming rate.

Many annual killifish from this stretch of Brazil are highly endangered. It’s very possible that within 5–10 years, some of these fish will disappear from the wild entirely.

That means the survival of species like Xenurolebias pataxo may depend entirely on hobby breeders.

If you work with this species, spread it around responsibly.

Temperament – Peaceful… Usually

Generally, this is a peaceful annual killifish.

But — and it’s a big but — occasionally you’ll get an individual (male or female) that turns aggressive and wipes out tankmates.

When first pairing them:

• Watch closely
• Be prepared to separate males and females
• Consider only introducing them together for breeding

Sometimes they coexist perfectly. Sometimes they don’t. It’s trial and error.

Breeding & Lifespan

This is an annual substrate spawner.

In the wild, their pools dry up yearly, so their natural lifespan is around one year. In aquariums, you might stretch that to two years with careful maintenance.

They mature quickly:

• Sexual maturity: around 6–7 weeks
• Extremely prolific once they start

To breed them:

• Provide a jar of peat or coconut fibre deep enough for diving
• Remove the spawning medium every 1–2 weeks
• Dry the peat and store it at 24–25°C
• Incubate for 2–3 months

They will produce hundreds of eggs once established.

Tank Setup

• Minimum: 9 litres (I recommend closer to 12 litres)
• Sponge filtration only
• Bare-bottom tank works well
• Plenty of wool mops and hiding spots
• Small weekly water changes (don’t overdo it during breeding mode)

Water parameters:

• pH: 6.5 – 7
• Temperature: 22–27°C
• They’re not particularly fussy about hardness

They are benthopelagic feeders, meaning they’ll take food from anywhere in the water column.

Feeding – Important Warning

They’ll eat almost anything.

But avoid feeding live or black bloodworm.

Multiple breeders have reported mass die-offs after feeding bloodworm to this species. Whether it’s bacterial contamination or digestive issues, I don’t know — but it’s not worth the risk.

Stick with:

• Brine shrimp
• Daphnia
• Grindal worms
• Quality frozen foods

Albino Fry – Separate Them Early

A large percentage of fry can turn out albino.

If you notice albino fry:

• Separate them early
• They grow slower
• They feed more slowly
• Larger siblings will outcompete them

If left mixed, they’ll gradually waste away.

Conservation Notes

There are four closely related forms within this group, all found in roughly the same Brazilian drainage systems.

This is a poorly researched group of fish, and habitat loss is accelerating.

If you’re breeding them:

• Keep records
• Share bloodlines responsibly
• Document growth, behaviour, and incubation data

Many species disappear before we even understand them properly.

This one may well rely on the hobby to survive.

And for that reason alone, it’s worth keeping.