How do you handle sickness?

Depends on what it is. But mostly shove any illness into their QT tank. That’s why I have 2 of them setup and ready to go whenever.

It depends on how contagious it is, but this is an area where I am still learning.

I think it depends on the illness - if it’s something that affects the whole tank (ex; ich/velvet) I’d treat the whole tank. If it’s something that is specific to that fish (ex; fin rot, head in the hole disease) I’d just take that species out and treat them.

Avery's advice makes sense to me. Just go based on context.

I ask my vet how it’s to be cured.

When I noticed something with one fish, I completely switch out the water and put the sick/ill-looking fish in quarantine. This way, it keeps it from spreading as easy while I figure out what is happening.

If only a particular fish is infected then it can be handled or treated better than when the whole tank gets infected.