How to Set Up a Killifish Tank for Semi-Annual Mop Spawners
This guide is based on the setup method shown in the YouTube video linked above. It’s designed specifically for semi-annual mop-spawning killifish — simple, cheap, and proven over years of breeding.
1️⃣ Tank Size & Basic Equipment
Ideal Tank Size
- 30 cm x 18 cm x 18 cm (roughly 12″ x 7″ x 7″)
- Larger or slightly smaller tanks can work depending on species
- This size is efficient for breeding pairs or trios
Tight-Fitting Lid (CRITICAL)
Killifish are exceptional jumpers.
Even the smallest gap is enough for them to escape — and they will.
Most shop lids have:
- Gaps for cables
- Open feeding holes
- Filter cutouts
These are dangerous.
Best solution:
- Cut a sheet of flexible plastic to the exact dimensions of the tank.
- Leave a small fold-back feeding notch.
- Ensure absolutely no gaps.
A single small gap = dead fish.
2️⃣ Heating – Usually Not Needed
If the room is warm and stable:
- No heater required.
Most semi-annual mop spawners do perfectly well at normal indoor room temperature.
3️⃣ Filtration – Keep It Simple
You only need:
- Small sponge filter
- Air pump
- Airline tubing
Sponge filters are perfect because:
- Gentle flow
- Fry safe
- Cheap
- Reliable
- Easy to maintain
They’re ideal for breeding tanks.
4️⃣ Advanced Sponge Filter Upgrade (Optional)
If you travel or leave fish unattended for a week or two, you can increase biological filtration using a simple DIY upgrade:
What You Need:
- Small screw-top plastic container
- Ceramic rings
- Sponge filter
- Drill
How It Works:
- Drill:
- One hole in the bottom (for airline)
- Multiple holes in the lid
- Fill the container with ceramic rings.
- Feed the airline through the jar.
- Attach sponge filter on top.
Water is drawn through the sponge → bubbles rise through ceramic rings → bacteria colonize rings → increased biological filtration.
Ceramic rings provide:
- Huge surface area
- Strong bacterial colony
- Long-term stability
This setup can run for weeks without maintenance.
5️⃣ Water
Use:
- Aged tank water
OR - Dechlorinated tap water
OR - Water left standing 24 hours
Nothing fancy required.
6️⃣ The Spawning Mop (Very Important)
The mop is where the eggs are laid.
How to Make One:
- Use dark wool (black preferred)
- Wrap around a book to create equal strands
- Tie at one end
- Attach to small piece of polystyrene so it floats
7️⃣ Mop Length Depends on Species
Killifish lay eggs at different levels:
🟢 Top Spawners
Lay eggs near surface → Keep mop short.
🟡 Mid-Water Spawners
Lay eggs halfway → Medium length mop.
🔵 Bottom Spawners
Lay eggs near base → Mop should reach bottom.
Important:
If not breeding bottom spawners:
- Keep mop off the substrate.
- Detritus collects at bottom.
- Dirty mop = infected eggs.
8️⃣ Why Use Black Wool?
Black or very dark wool is best.
Reasons:
- Many killifish prefer darker spawning areas.
- Lighter green mops are often ignored.
- Eggs are easier to see against dark wool.
Dark mop = more eggs.
9️⃣ Feeding Before Leaving (If You Travel)
If leaving fish for 1–2 weeks:
- Feed heavily beforehand.
- Add live foods like scuds.
- Ensure strong biological filtration.
Semi-annual species can handle short fasting periods if well conditioned.
🔟 Final Setup Checklist
✔ 30 cm tank
✔ Tight plastic lid (no gaps)
✔ Sponge filter
✔ Air pump
✔ Ceramic ring upgrade (optional)
✔ Dark wool spawning mop
✔ Aged/dechlorinated water
That’s it.
No substrate required.
No plants required.
No expensive equipment required.
Final Thoughts
There are many ways to set up killifish tanks — and every breeder has their own method.
But this system works because it is:
- Simple
- Cheap
- Easy to maintain
- Extremely effective for mop spawners
For semi-annual killifish, simplicity and stability are far more important than elaborate aquascaping.
