This is a super long shot............But research pest traps and methods.
Because something could be living in your tank for years now that is eating / P@ssing your corals off at night. And coral warfare even though most reefers denies it is a real thing!!!!! And by morning and or mid day your carbon has filtered out the excess toxins. Hence why you never pick it up on your tests. But your corals are still suffering and slowly withering away.
Its a super long shot. But in your tanks case we must explore and or at least entertain any and all possibilities until it has been ruled out.
Because something could be living in your tank for years now that is eating / P@ssing your corals off at night. And coral warfare even though most reefers denies it is a real thing!!!!! And by morning and or mid day your carbon has filtered out the excess toxins. Hence why you never pick it up on your tests. But your corals are still suffering and slowly withering away.
Its a super long shot. But in your tanks case we must explore and or at least entertain any and all possibilities until it has been ruled out.