Originally Posted by Delphina
This morning I found one of our black swordtails swimming rather unusually, twisting and erratic and had black stringy poop. Kept swimming up to the top and then just floating back down. I went out then came back and it was on the bottom with its fins flapping. Can anyone help why it would be doing this? We have other fish in the tank and don't want to lose them? Thanks
To me that sounds like some sort of parasite. I'm sorry to say that I think the infected fish is probably a goner, given what you've described. API General Cure is a good treatment for parasitic infections. You'll want to use it to protect the other fish.
Like I said, this is just a guess. Others may have better advice. Good luck and keep us posted.