Sick/dying guppies - diagnosing help needed

[Emily B]

Hello, guppy people. I'm looking for some help with an illness diagnosis. I've had a heavily planted community tank running for over a year with an all-male group of mutt guppies, an otocinclus cat, and some random snails. Water parameters are consistent, tested very frequently, and read as follows:
🌱 0ppm ammonia
🌱 0ppm nitrite
🌱 20-40ppm nitrate (dosed from Easy Green)
🌱 7.6 PH
🌱 11 GH
🌱 10 KH
🌱 Tank temp 75 degrees
🌱 Running internal filter + air stone
A few weeks ago, I got a new batch of 6 guppies, which I quarantined in a separate hospital tank. They received a 5-day dose of Maracyn, ParaCleanse, and Ich-X. After running the meds and observing a few more days, they looked great, and I introduced them into my main tank. After a couple of weeks, one of the new guppies was shimmying, swimming slightly vertically, and staying near the top of the water. I immediately pulled him and placed into the QT tank with salt and clean water. After 3 days, he passed away overnight. I elected to run another course of the med trio in my main tank to try and stave off any illness that might be beginning. Over the last week, several more guppies have presented as sick the exact same way as the first, despite soaking in a second run of the med trio, and more have unfortunately died. They have had:
👉🏼 zero signs of fin rot
👉🏼 zero signs of external parasites
👉🏼 were eating well and active before appearing suddenly sick and quickly dying

👉🏼 tails seem to droop slightly downward upon getting sick
👉🏼 no predation/aggression happening in tank
👉🏼 no red gills or abrasions on the body
👉🏼 slightly swollen bellies
I have pulled the remaining guppies and placed them in my QT tank so that I can observe more easily. The snails and Oto seem 100% unaffected by whatever is going on. It seems to be something infectious, perhaps a parasite or infection? Has anyone dealt with something like this before? I would welcome any guidance from experienced fish keepers. I can absolutely order different meds if anyone has a bright idea as to what this might be. I feel terrible that the quarantine treatment I ran clearly wasn't enough to keep something from spreading through my beloved tank. Thanks in advance for any help you may be able to give me. I'm very worried that the remaining guppies aren't going to make it if I can't figure out what is happening.

[Colu]

What your describing could be a parasitic infection what I would do is treat with paracleanse every two weeks for 3 full course of treatment following instructions on the box @Emily B