I'm sorry to hear about your bettas. 😢 To determine if your tank has a disease, it's crucial to look for symptoms such as unusual behavior, changes in appetite, spots, or discoloration on the remaining fish. Also, check water parameters like ammonia KH, pH, and GH, nitrites, and nitrates to ensure good water quality. Quarantine any sick fish immediately, perform a partial water change, and monitor closely. Consider reaching out to a local fish store or an aquarium expert for a more detailed diagnosis based on specific symptoms.