Dying Fish-Help please!

[Windmill]

Sorry for the delay--it has been rough.
here is what I have left in my tank-
3 cory catfish
3 pristella tetras
3 serpae tetras
2 danios
2 platys. I lost the two clown loaches within hours of each other, that was so hard on me and poor them. ALSO--the one danio when he died appeared to have blood around his gills. Just him. I don't know if that is significant or if someone could have bit him. The tank had been dark for a while, so it is hard to tell. I have a pic-of him-it is my first aquarium pic. So it is not good. If anyone is interested. I cannot seem to figure out how to attach it. I am not normally THIS blonde.
I have done a 50% water change, since the most recent deaths. The water temp is about 78 and all tests still show within normal range.
Thr remaining fish have not eaten while I have been watching. There have been no more deaths since Sat. at around 7pm.. So 18 hours or so and holding......
It is still a mystery and a real sadness. Wish I knew what happened so I did not do it again. Thanx for listening.....Jerri