A couple of things. Nitrite poisoning looks like a fish is suffocating, but this is not from lack of oxygen in the water. The most important sign of nitrite poisoning is that fish will be gasping for air at the surface. Here is how it works:
Brown blood disease occurs in fish when water contains high nitrite concentrations. Nitrite enters the bloodstream through the gills and turns the blood to a
chocolate-brown color. Hemoglobin, which transports oxygen in the blood, combines with nitrite to form methemoglobin, which is incapable of oxygen transport.
Brown blood cannot carry sufficient amounts of oxygen, and affected fish can suffocate despite adequate oxygen concentration in the water. This accounts for the
gasping behavior often observed in fish with brown blood disease, even when oxygen levels are relatively high.
from
https://srac.tamu.edu/serveFactSheet/110
So my first question is did you notice fish at the surface a lot or hanging out in the return flow from the filter? However, I would be surprised if nitrite affected only one species in a tank. Next, regarding the test result of 5 ppm. If you are using the API nitrite kit, it stops at 5 ppm, so you don't know if your actual level was 5 or maybe a lot more.
The white around the mouth is typical of columnaris aka "mouth fungus" some forms can kill fast. Have a read here:
http://animal-world.com/encyclo/fresh/i ... outhFungus
Again, This should likely affect other species, not just one. Akso, you saw the fish and see those remaining, so you need to decide if this might be what you are seeing.
Finally, the one thing you do not have numbers on is ammonia. If all your work wiped out half of the nitrifying bacteria, you can expect an ammonia spike which is why you have nitrite. However, between the time it happens and the ammonia bacteria/archaea reproduce to deal with it can be a day or less. So ammonia may have been the culprit. However, your plants sould contribute some amount of ammonia consumption capacity to your tank.
When it comes to any of the 3 nitrogen compounds in tanks, there is no universal level at which any given species will suffer or succumb. Different fish can tolerate different levels for different amounts of time.
If the problem were low DO I would also have expected other fish to show some symptoms as well even if they did not die. Those symptoms would be similar to nitrite poisoning. So I would ask the same question, did you notice the behavior described above?
This is about all I can think of at this point. Between Darrel's thoughts and what I have added, I think one of these things might be to blame. I share his disloke for internal filters.
You can deal with nitrite by adding chloride to the water. It will block the nitrite from getting into the blood. Adding a very small amount of salt will do the trick. I can give you instructions for doing diluted testing for nitrite if your kits still reads at its maximum level and you do not know how to do this.. You need to know the actually ppm of nitrite to know how much salt is required. I ran a quick estimate using 50 gals- your tanks never hold the amount of water they are labeled as. I assumed 10 ppm of nitrite and aimed to have 10 times the chloride in the water as nitrite. The amount of salt you would add would be 1 level tablespoon + 1 level teaspoon (about 29.1 grams). I use plain old table salt.