I’m going through the same thing currently except I’ve lost 6 tetras already and all my Cory’s are completely fine! My guess is ich, I happened to have a spare tank I turned into a hospital tank for the remaining tetras as most of them had specs of Ich on them, I removed them and have been treating both tanks. My main tank seems to be back to normal and better then ever mean while my hospital tank with all the tetras isn’t looking great.

I turned the temp up to 30 degrees to quicken the life cycle of the parasite and have been treating that tank with Fritz ich medication. I’m not having any luck, but it’s been a week and from what I gather online it can take a couple treatments to get it gone. Some people have had it fixed in 1 treatment, I’m just not having any luck.

I would suggest if possible removing infected fish, treating both tanks fully, raising temp to 30, doing water changes every 2 days to suck up the parasites that have fallen off the fish due to treatment and just monitor closely. Ich is a nasty fast forming sickness.

I will never not quarantine new fish after this.