Betta Fish: How do I Sex them? (16 week old Betta babies)

I shifted them to a smaller tank for better identification.

(Hatch date: 6 July 2025)

Behaviour:
So far no nipping behaviours or flaring, just absolute dorks.
In all honesty they look like they have nothing going on in their heads. Cute idiots.

So right now, I’ve shifted my 16-week-old betta juveniles into a 4-litre observation tank just temporarily. I needed to monitor their fin development and feeding behavior more closely, so this is purely for short-term observation before they go back into the main grow-out system.

🔹 Previous Grow-Out Tank (Main Setup)

Tank size: 20 gallons (around 75 litres)
Population: approx 30-ish? juveniles (now 16 weeks old)
Filtration: sponge filter (matured for 6 weeks before fry were added)
Heater: 100W preset heater maintaining stable temp
Lighting: 12/12-hour cycle with LED (no direct sunlight)

Parameter Reading Notes

Temperature 28°C (82°F) Stable, monitored with digital thermometer
pH 7.1 Slightly neutral, stays steady thanks to moderate KH
Ammonia (NH₃) 0 ppm Fully cycled sponge filters
Nitrite (NO₂⁻) 0 ppm Indicates mature biofiltration
Nitrate (NO₃⁻) 12 ppm Controlled with 40% weekly