Agree with Stephen and....
In many tropical countries, like where I live in Panama there are a couple seasons, not as obvious as in northern countries but significant no less.
There is the rainy season where water temps drop (especially in places where rivers that flow down from the mountains, like the Andes in S America, the Sierra in Mexico, Himalayas in Asia, etc) lower average temps, and the dry season, where water temp rise.
And as Stephen said
ammonia and nitrite are very stressful at even tiny amounts, and cause a fishes immune system to be compromised, allowing even normally non-lethal bacteria to take advantage, and become infectious.