Mysterious Death!

So, did a water test earlier this week, plus did a water change and everything is fine...then did another test just earlier today for the nitrates because of a spike I had experienced not too long ago that caused two deaths, which was taken care of with come water changes. Here it is over a week later, I take my eyes off them for not even an hour, and I come back to give them their dinner and one of my bloodfins is laying belly up in the bottom of my 45 gallon. Upon closer inspection of the body, since their skin is pretty see-through, I could swear that there is blood on the inside. I could be wrong however...I'm not completely sure.
The now past one shared the tank with 3 other bloodfins, 3 Blackskirt tetra, and 3 cory catfish along with a few live plants, so by all means they have a ton of room unless I have a bully in my tank....Any thoughts...?

 

Can you provide your exact water parameters - pH, temp, ammonia, nitrite, nitrate?

 

The ph was a little high but nothing that couldn't be fixed, nitrites were where they should be, I upped the temperature to around 78 to help with any ich, and I just did a change for the nitrates Thursday and tested them today, it wasn't even 0.25...unfortunately I suspect one of my blackskirts of bullying...I use to have a pretty longfin until I caught it butting her around too late. I could be wrong though. Maybe I should test again

 

There's no such thing as testing your water too many times, try and move it around the tank a bit too because you might develop spots or pockets, but I suspect the bullying is more likely so it's best to keep an eye on the behaviour of all your fish towards each other over the coming days to note whether a problem fish needs quarantine from the others

 

The ph was a little high but nothing that couldn't be fixed, nitrites were where they should be, I upped the temperature to around 78 to help with any ich, and I just did a change for the nitrates Thursday and tested them today, it wasn't even 0.25...unfortunately I suspect one of my blackskirts of bullying...I use to have a pretty longfin until I caught it butting her around too late. I could be wrong though. Maybe I should test again

Ammonia and nitrite should always be zero. What was the exact pH? What are you testing with? The lowest reading for nitrates using the API master kit is 5ppm not .25.

If you were treating for ich using the heat method, it would have needed to be raised to 86F for two weeks. Are you seeing white spots on them?