Neon tetras dying for no apparent reason

Tank
What is the water volume of the tank? -55 gallons
How long has the tank been running? -Since September
Does it have a filter? -Yes
Does it have a heater? -Yes
What is the water temperature? -78.6 *F
What is the entire stocking of this tank? -Two angel fish (they leave the tetras alone from what I have observed), 10 danios, two very small baby goldenwonder killifish, 9 neon tetras (was 15 before all this)

Maintenance
How often do you change the water? -Up until two months ago it was once every three weeks, since then its once every four or so weeks
How much of the water do you change? -25% give or take
What do you use to treat your water? -Nothing. I have well water and it is purified. I have many other tanks and they are all very healthy
Do you vacuum the substrate or just the water? -I do vacuum the substrate

*Parameters - Very Important
Did you cycle your tank before adding fish? -Yes
What do you use to test the water? -API master test kit
Ammonia: 0ppm
Nitrite: 0ppm
Nitrate: 20ppm
pH: 7.4

Feeding
How often do you feed your fish? -Once a day
How much do you feed your fish? -A few pinches? I do not know how to measure this. It is enough to feed them
What brand of food do you feed your fish? -Vibra bites
Do you feed frozen? -Once or twice a week
Do you feed freeze-dried foods? -Occasionally bloodworms

Illness & Symptoms
How long have you had this fish? -Five or so days
How long ago did you first notice these symptoms? -Three days ago
In a few words, can you explain the symptoms? -Neon tetras are just dying for no apparent reason
Have you started any treatment for the illness? -None yet
Was your fish physically ill or injured upon purchase? -Nope
How has its behavior and appearance changed, if at all? -They might be being more aggressive with each other, but it does seem like normal behavior.

I got these fish on Tuesday and put them through this rapid quarantine product (I have found that it works well in the past, I will attach a picture) and had these tetras in a floating breeder box for a day or so to get the fish in the tank used to them. They are now just dying off. Only one has had any sigh of injury, that being a small cut. It was not eaten or anything, just a small red cut. It does seem like the smaller tetras are the ones that are dying. The nitrate levels have suddenly increased, might it have anything to do with that, or is it that they are not used to hard water? No matter what, I will be doing a water change before I check back here.
I found yet another one while doing my water change, I might have missed this one earlier. It has a red spot on him, and something similar on this other side. Is it just aggression, or is it something worse?

 

It appears these fish came sick. They may be in septic shock now. I would remove them and take them back. Explain to the store what is happening. The rest of their stock is probably sick too. They should owe you some healthy fish.

 

Check out the posts from chromedome52 at Are neon tetras easy for beginners? | Neon Tetra Forum.

 

Ok, thank you for telling me. Should I do anything to the tank itself to protect the rest of the fish?

 

The safty stop is 2 medicines to treat fungas and parasites. It should not be called a quarantine, it is not a quarantine, it is a preemptive strike against possible illnesses. It is still a good idea to do a fulll 2-4 week quarantine to protect the other fish in the tank.
Formaline and methaline blue are both a little bit hard on fish. Neons are sensitive fish. I dont know for sure, but the medicines, alonge with being netted to the two different buckets, and the netted a forth time in one day to the home tank, I might have been too much.

I would turn the tank lights off, keep noise and activity around the tank to a minimum, maybey even put a towel over the tank.