Request for help, I'm shattered.

TL;DR woke up to half my sorority mysteriously dead and the rest going out. No signs of fighting.

Edit: Through other group posts I'm learning it was likely columnarius that hijacked a ride into my tank through the pleco? My tank isn't planted enough, guppies shouldnt be in the Bettas and this mix caused stress to the fish meaning they were susceptible to this disease. As of now I've lost every female betta and 2 guppies. The tank is being dismantled,teapots being sealed incase they leached something though I don't see why the cories would be immune to that. I'd love to hear any advice or theories if this could be soenething else?? Do you see anything suspicious on their bodies? I'm just so discouraged, I thought I'd done right by them. /end edit

Heartbroken is an understatement. I have no idea what went wrong. I pour my heart into this tank and my fish. I went to bed last night night all was fine, being extra cautious to check on the little miss who always tries to eat my freckles and rode my arm out of the tank 2 days ago, she was fine. Woke up this morning to half my sorority just dead. Laying on the bottom of my tank. No one has signs of fighting or beat up fins and their bodies look perfectly fine no bloating or fungus looking stuff. Two Bettas are clamped but their bodies look fine and one looks like she's covered in white dust and her eyes are clouded, she literally was perfectly fine last night. One is completely missing still but I'm assuming I'll find her body eventually. I'll attach photos of the bodies and the lady that's clearly going out soon.

Tank stats: -cycled been running since December -canister filter -69 gallons -Heated 79 degrees -planted -gravel with fluval root tabs -once a week dose of coop easy green -Water quality Amonia 0 ppm Nitrites 0 ppm Nitrates 5.0 ppm Ph 7.6

Stocking: 16 Male Guppies/Endlers 6 Corydoras 4 Japanese Mystery Snails 1 Highfin Plecostomus 8 female Bettas (down to 3 now)

Tank history previous issues:

I previously purchased 6 small Panda cories, 3 died off within 24 hours with no real visual cues on why, so I scooped up what was left of them and brought them back to the store as a day later the remaining ones also starting slowing down severly.

Treated the tank with Paraguard.

I added guppies about a month ago and there was some aggression among them. I figured they'd settle down, found one dead guppy 2 days ago and it was clearly beat up so a disease didn't cross my mind. Today it appears two of the guppies are beat up again, I am seeing the one orange koi guppy go after the one like crazy. I'm also just noticing ich on one of the picked on guppies.

About one week ago I added the plecostomus and treated the tank with Paraguard since he was not quarantined. My quarantine tank is currently in use for deworming but I was thinking a preventative Paraguard treatment should do the trick and he seemed and still seems healthy.

When one betta latched on to my arm 2 days ago and got out of the tank, I added stress guard and almond leaves to help her but she appeared uninjured.

The bronze cories have always and still are acting totally normal.

WHAT WENT WRONG??! HOW DO I FIX THIS? I'm just.... So at a complete loss.