Unknown disease spreading quickly through my fish?

[Antichton]

I suppose this is a lesson in quarantining new fish....

So I recently bought a few clown killifish from the LFS. They came in super tiny (around 1/4" long). I drip acclimated them to my water and put them in a breeder box so they wouldn't get eaten by my larger fish. One week later, 4/5 I bought are doing great (I lost one to a swim bladder issue that developed after 3 days). I was pretty happy with the ones I bought and wanted to round out the school, so I went back to the store and bought 6 more. 1 week later, and I've lost over half of the killies. To my eyes, they look fine on one day (eating and swimming normally), and then I wake up the next day to find 1 or 2 suffering from a swim bladder issue (either floating at the top or wriggling at the bottom). The fish with swim bladder issues died the next day. 

Normally, I'd just think of this as a bad batch or weak fish, but 3 days ago, one of the dying clown killies slipped through the grate in my breeder box and one of my gardneri killifish ate it. 24hrs after that fish ate the dead fish (2 days ago), it became lethargic and a 2hr after that started suffering from swim bladder issues. Furthermore, 2 of my 2yo clown killifish school and 3 of my guppies have died under similar circumstances over the last 2 days(became lethargic, started swimming at water surface, rapid gill movements, leading to death by nighttime). These fish did not come into contact with the new clown killifish. I don't notice any white spots or external parasites on any of my fish, and my remaining fish seem active and healthy. 

I've honestly been blown away by how quickly this disease/parasite has gone through my fish. I'm quarantining any fish that seem lethargic or have rapid gill movements in a 2.5g bucket dosed with praziquantel and methylene blue, and have dosed my main tank with praziquantel after doing ~50% water change, but I'm not sure if it's helping yet. Any ideas about what could be happening?  

I've attached a photo showing 2 "lethargic" fish who appear to be in the early stages of the disease. Here's a video showing the difference between how they behave and how the other fish in the tank behave: https://imgur.com/alCiJcR. 

TLDR: Fish become lethargic, rapid gill movements and swim bladder issues appear within 10hrs of first seeming lethargic, death follows within 24hrs. No apparent external parasites. Did ~50% water change and treated tank with Praziquantel. Any ideas about what is causing this/how to treat?

Here are my water parameters, tank has been setup and running for ~10years:

Temp: 75-80F depending on time of day

pH: 7-7.2

GH: 200ppm
KH: 80ppm
Ammonia, Nitrite: 0ppm

Nitrate: 20-40ppm

[Colu]

Have your fish had a sunken belly or white stringy poop also it could low levels of desolved oxygen I would add an extra air stone @Antichton

Edited August 20, 2021 by Colu

[Antichton]

Not that I can see; most of my fish are still eating well. I'd take a picture but no one is pooping right now....

[Colu]

How quickly your fish are dieing it could be an internal bacterial or internal parasites what I would do is a course of kanaplex in food  after that I treat for parasites as you have praziquantel I would also treat their food he's a food recipe for kanaplex and praziquantel

Edited August 20, 2021 by Colu

[Antichton]

On 8/19/2021 at 6:46 PM, Colu said:

Have your fish had a sunken belly or white stringy poop also it could low levels of desolved oxygen I would add an extra air stone @Antichton

Hmm, I would expect to see more than just a couple isolated fish at a time displaying symptoms if oxygen is low though, right? I don't have any airstones in the tank, but I do have 2 HOB breeder boxes that are driven by air. I can turn up the air flow on these boxes. 

On 8/19/2021 at 7:00 PM, Colu said:

How quickly your fish are dieing it could be an internal bacterial or internal parasites what I would do is a course of kanaplex in food  after that I treat for parasites as you have praziquantel I would also treat their food he's a food recipe for kanaplex and praziquantel

Alright, I'll give this a shot!