When you know that any of your fish is sick, the first major step is to quarantine the sick fish, probably in a separate tank or bowl. Because the other fish will sense the weakness and will pick on the sick fish.
What you said is perfectly true. Always separate sick fishes, as they need special care, attention and diet to get better. Examine the reason for their sickness and provide suitable antibiotic course. Check the ph level of the isolation tank and see if the water is aerated optimally and also check for the water temperature. Your fish will get better soon and you can release it to your main tank again.