[Esahc]

I recently set up a new 20gal planted tank with endlers and cories. I quarantined the cories while the tank was cycling, but like a moron (this is a recurring phenomenon with me.) I did not quarentine the endlers, because I was too excited to get them in the tank, and because the quarantine tank's filter wasn't cycled yet, and I was worried about the ammonia levels.
Since then, of the 6 endlers I started with (with plans to add another ten in the next week), they started dropping off over the last two days until two were alive this morning, and about five minutes ago I just watched one swim erratically in zigzags across the top of the tank and then die.
the other has stringy white poo, and I'm cleaning up the quarantine tank to separate him again. A little research is making me think this is neon tetra disease?
Does anyone have any experience with this disease? I'm concerned about the cories (they all seem healthy and active right now, they're swarming over the substrate and walks of the tank and are eating well with no signs of clamped fins or distress will they be affected or are they okay), and I'm concerned about how long I should wait before trying to introduce more endlers or neons into the tank.

diagnosis form:
1. Size of tank 20gal

2. Water parameters
a. Ammonia tba
b. Nitrite tba
c. Nitrate tba
d. pH, KH and GH? ph tba kh/gh n/a but my local water source is very hard.
e. Test kit Api freshwater mastertest

3. Temperature? 79F

4. FW (fresh water) or BW (brackish)? Freshwater

5. How long the aquarium has been set up? Initial setup was ~july 3rd, cycling began on the 7th and I got impatient and added marineland's aquarium bacteria on the 17th fish were added the evening of the 18th

6. What fish do you have? How many are in your tank? How big are they? How long have you had them?
5 false julii cories
1-2 endlers guppies
the cories were bought around three weeks ago with no sign of illness, the endlers were purchased last thursday.

7. Were the fish placed under quarantine period (minus the first batch from the point wherein the tank is ready to accommodate the inhabitants)?
the cories were quarentined for about two-two and a half weeks, the endlers were not.
8. a. Any live plants? Fake plants? live plants
b. Sand, gravel, barebottom? soil capped with fine gravel
c. Rocks, woods, fancy decors? Any hollow decors? driftwood

9. a. Filtration? tetra whisper 40 with the intake baffled and the flow turned to low
b. Heater? 100w

10. a. Lighting schedule? What lights are used? 1 t8 daylight bulb ~9-12 hours a day not on a timer
b. Any sunlight exposure? How long? insignificant amount from a nearby window with blinds drawn

11. a. Water change schedule? none yet. one 90% before fish were added
b. Volume of water changed?
c. Well water, tap water, RO water? treated tap water
d. Water conditioner used? api stress coat
e. Frequency of gravel/sand (if any) vacuumed? none yet

12. Foods? omega one flake for the endlers, veggie rounds and shrimp pellets for the cories
How often are they fed? endlers, a few flakes every morning. cories get a round every other day and the occasional shrimp pellet. they also seem to be cleaning up the brown algae from when the tank was cycling.

13. a. Any abnormal signs/symptoms?
b. Appearance of poop? stringy white poo from the endlers
c. Appearance of gills? normal

14. a. Have you treated your fish ahead of diagnosis? no
b. What meds were used? none.

15. Insert photos of fish in question and full tank shot if necessary.

I'll edit with the water parameters soon. yesterday they tested 0 for ammonia and nitrites, but I haven't tested nitrate since I introduced the fish. I'll also be doing my own research, but I wanted to get this thread up quickly in case anyone else had advice for me.

 

[Esahc]

It won't let me edit but here's an update on the situation and the water parameters.
Ammonia 0
Nitrites 0
Nitrates ~5

that was before the water change. I just did ~80% water change and vacuumed the gravel as well as I dared without stirring up the soil underneath.
I've separated out the last endler that I could find, but by my count there's still one in the tank.
I started with 6, one died in quarantine that I blamed on the ammonia levels by friday. One was dead in the main tank the next morning, saturday, sunday another was dead and caught in an amazon sword. sunday night, I could only find two, but as I turned of the lights I found the third. this morning the fourth one died as I was watching, and I have the fifth one separated out and going back into quarantine. I can only hope that the sixth one is still alive and just hiding very well as I've just gone through the entire tank and moved all the driftwood I can move looking for him with no luck. if I do find him, I'll separate him out as well and keep both current survivors cupped and floating in quarentine for now.

Cory cats are still active and healthy, chowing down on what I'm assuming is the brown algae that showed up in the tank during cycling(it carried over from the brown algae problem in my betta tank, which is where the gravel came from that I used to seed the filter.), none of them are showing any signs of distress or illness that I can see. I'm hoping I don't lose the whole tank over this.

 

[Esahc]

I wish the edit button would stop timing out.
One last update for the night.
a little more research makes me think this might be Hexamitiasis instead? None of the dead fish showed the fading color or any other symptom while they were alive, and from the one I watched die, I think the bodies only went pale after death. I'm really hoping that I misdiagnosed the first time, as hexamitiasis looks to be more treatable than NTD. And white stringy poo looks to be a general symptom for internal parasites?

Should I try a general parasite treatment in the main tank as a preventative? I have some left over from a similar situation with some platys about a year ago that ended with a lot fewer fatalities.