I have a ten gal, fully cycled tank. 6 albino corydoras (used to) inhabit it. All params are normal. It is moderately planted with sand, and the plants are doing well. Temp's fine. Now what is happening to my fish?

Three days ago, I noticed one of my albino cories was super pale...he had barely any colour left in him. Like, you could only see a small amount of pink. It was almost as if he had been drained of blood (figuratively of course). No signs of disease or parasites, no injury. Just really lethargic. Well, I did a 50% wc. He ended up dying the next morning anyway.

Now another cory is exhibiting the exact same symptoms. What do I do?? I hesitated to dose with Pimafix with the first fish because the others seemed healthy and I didn't want to medicate them as well...but now it seems that it's something contagious.

Please help. I don't have a quarantine tank. Yes I know, stupid. Anyway, should I dose the meds? They're all I have at the moment. What is killing my fish???