@JVH There are only two diseases (I know of) capable of
NOT showing visible physical symptoms in a 4 week time frame: ich & gill flukes. Both can harbor inside a fish's gills, out of sight. However, the following
key behavioral symptoms should still be present: Heavy breathing, reduced or complete loss of appetite, rubbing, flashing, head twitching, yawning, erratic swimming behavior. Unfortunately flukes & ich in the gills share these same symptoms, so you either have to treat for both or perform a FW dip to confirm/rule out the presence of flukes.
A fish with internal parasites will usually have white stringy poop. Of course, practically speaking whether or not you notice something like white string poop or the behavioral symptoms described above depends upon two factors:
- How much time you are willing & able to spend observing your fish in QT everyday
- How attention to detail minded are you (and also just how good is your eyesight)
My wife tells me all the time I can't see what a good thing I've got, so I take that to mean I must not be a very observant person. Therefore, I play it on the safe side and prophylactically treat all new fish.