This can be misleading because I actually just had that experience. Went and bought a clownfish from the lfs and it was swimming super healthy with 10 other clownfish in the tank and none of them were showing any symptoms or lethargic.. after bringing the clown home and qt a few days later he broke out with brooklynella. Every sign at the store pointed to a healthy fish but as mentioned in other posts. Diseases can lay dormant. My worst fear would be if something ended up in my DT with a hidden velvet or something worse.
