eDNA testing is always an option. Won’t solve your current issue, but will at least tell you if anything is lurking in the tank. Aquabiomics offers this service. The service does take a while to get results. IME Foxface’s get stressed very easily. And stress can kill. Not saying that’s what’s happened. Could be a number of things. I would personally never trust anyone to quarantine besides myself. Not even reputable vendors that sell “quarantined fish”… Haha. Hope you get things figured out!