Sudden Bloodfin Die-Off

Hello, I am new here, but not new to fish. I woke up this morning to a gruesome sight: 9 of 10 bloodfin tetras dead, who had been alive the previous day, and the last one quite unwell. I have kept fish for over 20 years, and have never seen anything like this before. I don't know, maybe I've been lucky, but the worst I have ever had has been ich.

The concerning part is this is in my main 60 gallon tank, that I have not touched (save for water and filter changes) in 10 months. No new fish have been added, one death of a 10 year old scissortail rasbora about 3 months ago.  The tank itself has been runing for a year.

What worries me is it seems to be rapidly spreading to my danios, but no one else. Other tankmates include: 3 columbian tetras, 3 bleeding heart tetras, 4 scissortail rasporas, 2 cory cats (oddly enough there are 7 more of them in my QT tank to add) 12 black neon tetras, 2 neon tetras (also about 8 more to add in QT, had more but my 12 year old albino argentianian tetra ate all but 2, he has now been banished to my krib tank), and about 14 danios.

Symptoms are frayed fins and white patches/spots randomly with varying sizes (not ich, I have seen this many times, very different, larger spots, fewer in number, not uniform). Gills are not inflamed, scales are not protruding, breathing is normal, the infected fish are scratching on the rocks (like ich).

I have added "quich cure" for parasites, "melafix" for bacteria, and I can go get fungus meds if I need to. Carbon filter is off, airater is turned up. Temp is 78* F. My water is hard, being in the midwest. My other parameters are:

Nitrate 70

nitrite 0

Alkalinity 250kh

ph 8

I know my nitrates are kinda high, I was going to do a water change this weekend when the white horseman stopped by my tank.

I do have a 20 gallon QT tank going, but I have been a zookeeper and keep everything separate to a fault. I wear gloves when I work with that tank, it has it's own water bucket, it's own filter, it's own fishnet, and I always take care of it last and wash my hands with hot soapy water after.  None of the QTfish, or either of my other 2 tanks have this

So, I am asking for help identifying what this is, and how to treat it. I will be doing a 50% water change in 3 days from now. It really hit the redfins hard, but what confuses me is that it is not hitting any other tetras, but rather the danios. Is it bacterial, viral, fungal, or parasite?  Thanks for your help.

If they are flashing, chances are it's parasitic. But combo with bacterial is common due to the parasites making wounds. Can you post a picture?
Ich shouldn't kill that fast I would think and ich is easy to identify.

Well, the last bloodfin just passed.  In the process of getting him out, i flushed out one of the hiding bleeding heart tetras.  He seems to have white patches on the edges of his operculum (gill cover) and along the edges of his fins.  I will try to take a pic, the danios have it the worst and they are still zooming around like normal, with the flashing.  What confuses me is that besides normal cleaning, I have not done anything to this tank in 9 months, so I have no idea where this stupid parasite/bacterium came from. I will continute to treat with antibacterial and antiparasitic.  Now where is my poor camera...

What is the ammonia level in that tank? Also, are you using test strips or liquid test kit?

Most bacteria and parasites are opportunistic pathogens and they are always in the tank. Something has caused weak immune system response and the fish get sick so you need to find the root cause as well.

Sorry, Ammonia is 0, skipped over that one by accident.  here are 2 pics of the BH tetra, the danios were moving too fast to get a pic.

Honestly, I can't see much from the pictures but it isn't ich for sure.
If they are flashing/scratching themselves off objects, it could be a parasite of some kind more likely

With a KH / alkalinity level of 250 mg/L you probably also have an inappropriate PH level since your KH level can influence, indirectly, the PH.
 
A high KH typically causes a high alkaline PH which results in alkalinosis which causes the following symptoms in fish:
 

1. Flashing / rubbing
2. Jumping out of the water
3. Excess mucus
4. Gill damage

Please re-test your PH, if it is near 8.4, or particularly 8.8, you need to do a series of 20% water changes to lower it.
 
As the fish will feel most probably sun burnt from this you need to watch for opportunistic parasites taking advantage of skin cracks/soars, although this doesn't look like the cause of your problem at the moment.