I only have one API test kit, maybe I can get it analyzed at a pet store? I can also see if I can do another test where I test the water in the display and in the sump to see if there is a difference.If you haven't added any fish in 5 months, the chances that a disease will be latent for so long and then kill all of them in 2 days is almost zero. Can you double check the ammonia with another test kit?