First, you cannot trust a store or dealer to QT your fish....you must QT new fish at your own home to REALLY insure they are healthy. All fish have low levels of parasites....that is just a fact of life. A normal, healthy fish has an active immune system which keeps parasites from becoming a problem. When a fish is stressed, the fish's resources shift from maintaining the immune system to overcoming the stress. this leaves their immune system open to attack. Probably the largest stressors in a fish's life can include being chansed around, netted, bagged and moved to water that they are not used to (different temperature, pH, hardness, etc). So fish is now stressed....and immune system drops so the fish can survive the stress. These shubbies possibly either had parasites to begin with, or they may have picked them up from your fish, but at any rate the parasites can now plan an all-out attack on the fish because the immune system is busy recovering from stress. End result....new fish die and old fish are just fine.

I don't know for sure that is what happened, but it makes a lot of sense to me. Other possible cause is temperature shock or pH shock. Birds didn't kill them, neither did the old fish or frogs. Sorry you lost them though.