I wish I could tell you a for sure way but i dont know one. I like to back track and see if anything is leaking into the tank or in contact with areas of exposed water amything like that. could an electronic be broken and leeching metals into the water etc. i would just take a good in depth review of whats in or near your tank. or if theres any way a non save substance could have entered your tank like soap for example.Any idea what kind or how to find what’s causing the problem?