Small Tanks are hard and none forgiving. I'm sorry for your loss. I would of giving you 15 gallons from my tank that way you could just dump the bad water and giving you a chance. Winter has some challenges for tanks special ones with SPS. Lets See Heating is one PH is my favorite I just had a PH drop and lost some corals. Not paying attention because of the holidays. Leaving the the door closed and over heating but its all part of the madness. Good luck and Have a positive out look in the New Year.