Fish died after medication and recovery

[JaredShope1]

My fish was very sick and was laying on his side. I isolated him into a 5 gallon hospital tank (he is a molly). I medicated it with anti bacterial and anti fungal medications and he recovered very well. He was eating and swimming relatively normally. Then the next morning, he was just dead. I checked the water and everything was in check, 0 ammonia, 0 nitrites and 5 ppm nitrates. PH was 7 and GH and KH were at normal levels. Any idea why he was doing so well for 2 days then just suddenly passed away? Thanks