Female Betta Found Dead

I checked my tank this morning to find one of my female bettas floating near the bottom of the tank with her head facing towards the substrate. At first I thought she was eating something off the sand but then I realised she was just floating rather than moving. It was only when I looked closer it seemed as if she had what look like 2 red marks/cuts near her mouth. I've had 7 female bettas and 1 male in the tank now for around about 6+ months and they never ever fight. Sometimes 1 or 2 certain females will flare up to scare 1 of the others away from their territory but it's never vicious and more often than not you'll get 3/4 of the females swimming round together with no chasing or flaring what-so-ever and I've not seen the male betta flare once in months. I recently done a large water change though along with rearrange the tank/add a new large cave and I also added 2 new plecs and 9 kuhli loaches. The problem I have is working out what could have caused the death? All the other fish seem fine and don't seem/look any different at all. Just wondered if anyone knows what could have caused it?

Bless Her. Sorry for your loss.


How many gallons is the tank?
Full stocking list?
Water stats in ammonia, nitrite, nitrate, and ph?

If you want to keep the make betta I suggest buying a tank of his own.
They can kill female betta.

The fish that died did the marks look like injuries, bleeding, bleeding beneath the skin.