Internal parasites worry me a lot less since it's so easy to just soak some food in prazi and take care of the problem even in the display.
The more pressing issues why IMO quarantine is a requirement and not optional (I even QT mandarins) is because of the following
1. Place of no competition to get fish accustomed to your food and be less stressed
2. Place for stress to reduce before tankmates can pick on it and lead to its demise.
3. Place to treat and/or observe before something serious like velvet, brook, or flukes that can wipe the tank in 2 days and kill most of your coral because of the ammonia spike before you have a chance to treat
4. Easy place to dispose of fish without causing ammonia spike that could become a tank crash in the event the fish dies from any cause including the above.
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