Could you post a picture of the unidentified object?
Could you post the results of ammonia, nitrite and nitrate tests?
The most likely culprit is ammonia poisoning. After moving a tank the system is often shocked and can not process ammonia into nitrate as quickly as it did before. In a large tank with only two fish, it may have taken a couple weeks for the ammonia to build to a toxic level. Water changes will help this condition, you want to keep the ammonia below .5 ppm