Johanna,
Your problem fish here maybe suffering still from the aftermath of your deworming it. When a fish has worms the worms can burrow into tissue and organs. These worms then die when you treat them with a dewormer that can lead to localized infections and just not really doing well heath wise. There can be a period of time when the fish is just really off their game while they heal. I can't say 100% that this is whats happening but its one scenario that fits the time line and what you are seeing. You do want to watch for swollen abdomens and odd colored feces as that could mean theres a more than minor infection.If you can separate the fish to a bare hospital tank if will be easier to monitor it.. if not a partition in the QT tank may help.
al