I just purchased 8 kuhlis >48 hours ago and all of them are normal except one. He was laying on his side when I got home and then he had a seizure and stopped moving. he was still breathing after the seizure and then he had another one and passed away. Normal parameters (0 ammonia, 0 nitrite, >20ppm nitrate) and ph of 7.6-8 (they are captive bred kuhlis). Any explanation or advice?