It depends upon the disease. For example, ich can be treated by raising the temperature to the low 90's for a few days.

If medication is required, I think it is best to put the fish in a separate tank and medicate them there, although as you implied, catching them can be difficult in a heavily planted tank. Using two nets can make that easier, and in the process you will discover which plants aren't well rooted <g>.

Diseased fish also can recover on their own, without medication, particulary if the remperature is raised a little.
I have never used any medications, and my fish die of accident or of old age.

Bill