Internal parasites are contagious.. it's better safe than sorry to treat the whole tank.
If your fish have internals, they will die if not treated.. if they don't have internals.. no harm in treating anyways.
I am not trying to be rude, but I don't understand why people try to cut corners?
HOLDDD UP. No harm in randomly dosing a tank with medication? I beg to differ. Dosing a tank without knowing the cause for sure does many things, stress out fish and diseases/parasites can grow to be immune to the antibiotics.
@OP: I suggest you take your Red Terror and put it in a small quarantine tank and treat it with metro.. It sounds like it really does have IP's. Good luck.
You can beg to differ all you want, as this is a public forum, but your logic makes no sense. If a fish is sick, it's already stressed, so wouldn't adding meds just cause more stress? If the fish is healthy, adding a medication isn't going to stress a fish out. Yes, diseases/parasites can become immune to meds... but with that logic, what would be the point of even treating said sick fish? It's just going to become immune to the medicine.
QTing the fish will treat THAT fish, but what happens when all the other fish end up having IPs? OR if she doesn't treat the other fish, and puts the fish back in that tank after QT, it's just going to get the IPs again.
So, why the heck would you cut corners? It's just going to end up a big mess, and you will end up with more hassle in the long run. Treat everything once, and be done with it.
The FH in the same tank died, now the RT is not doing well.. chances are whatever the issue is; it's contagious, and while you don't recommend treating the whole tank, I do. Would you rather cause your fish a little stress, or just have them all die because you didn't treat the entire problem?
But, OP, it's your call..
The line I colored Red, think about what you just wrote.
Why treat the whole tank when only 1 fish is showing symptoms.. If anything dosing the tank will stress the healthy fish out. And yes, if you put meds in a sick fishes tank, it will stress out some more, but if you're dosing it properly(treating the right disease) then it would be fine.. and as the OP stated, his other fish couldn't handle the meds because it was so weak due to being sick. It happens. Now, question my logic all you want, Idc, but I'm letting the OP know what should be done.