There are lots of possible reasons, amongst which some are:
Perhaps the fish were already suffering some disease when you bought them;
Perhaps the water in the tank was poor quality (very high chlorine or pollutants in the water);
Perhaps the water temperature in your tank was too different from the previous tank;
Perhaps the fish were overstressed in the journey from where you got them.
Here is a helpful link: http://www.myaquariumclub.com/your-a...ssful-564.html
Bookmarks