Female Bettas Dying

[Biulu]

I bought 9 female bettas and slowly they are dying off. I am down to 4 now. They are not showing any outwards signs in terms of wounds, stress stripes or other. They eat, but become very apathic, with their tail fins closed (not clamped).

Yesterday I noticed white stringy poop coming out of one of them, and I dosed with praziquantel 2 mg/litre. Half an hour after the dosing I found 1 betta like this:

As you can see, the fins are intact so now significant tank mate agression. I had found another betta the other day where the red area was completely 'eaten' away. The red gills don't appear until death. There is no ammonia issue as I have 3 panda cories in this tank which are out and about and as you may know panda cories are pretty sensitive to water parameters.

I have just dosed praziquantel again as I still saw white poop hanging out. Since I am using praziquantel in tablet form, which I crush and dissolve in water, I am not sure whether this is as effective as the liquid form.

Does anybody have an idea of what this is, and how I can treat it?

Thanks

 

[Biulu]

Anybody? *Bump*

 

[Cazgar]

How big is your tank? Did you cycle your filter before adding them, as if you didn't then it could be ammonia poisoning

 

[Biulu]

It is not ammonia poisoning as my panda cories are absolutely fine and these fish are very sensitive towards water quality. Further, the fish don't show any red gills until they die. Today I found a lot of expelled tapeworm, did a water change of 70% and medicated again as there is still a fish that is showing white stringy poop.

Two seem to look somewhat better, nr. 3 I am not sure she will make it.