It's hard to say without some basic information. This separate tank... did you just set it up? did you allow it to cycle? It's usually best to keep the female in the main tank in a breeding box for 24 hours or less. If you have a water testing kit, and the tank where the female died in is still set up, let me know what the water parameters are so I can help you figure out what's wrong.
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Phishie

"An ounce of prevention is worth a pound of cure."