The low GH is no good for the endlers, they need a GH of 15 DH to stay healthy.
Having them in soft water affects their immune system. While this will not kill them outright, it leaves them prone to things that may kill them fast, such as bacterial infections, fungal, etc.
I struggled with this with guppies and they'd die within days of getting them, a few showing signs of acute columnaris infections when they were originally healthy in the store.
Learned my water is only 5 DH or 89.5 ppm, which is soft.
This affected my livebearers, while it didn't kill them immediately.
Now, I keep my guppies in a 50g tote and use Seachem Equilibrium to bring my GH up to 250ppm for them. Much healthier and all my problems are better.
However, depending on your corydoras species, they may not thrive in the high GH, so you may need separate tanks should you want to keep both.
So your low GH is a contribution to the problem you are having, so its something to work on should you want to continue with the endlers afterwards. So thats one thing solved, let's work on the rest to get these guys in better health.
thank you so much for the info, i'll def get a testing kit soon and work on getting the water right for endlers if this guy makes it or if i get some in the future (and ill make sure it works for the corydoras). idk what to do with the guy i have right now tho, i will check on him in the morning and see if he's still alive. he was alive as of a couple hours ago.
is there anything i could add to the water besides a product (like a type of rock or shells or smthn) to help get the water right?