Your fish may not have died from any of the below diseases but you need to understand them and recognize their symptom because they kill quick and infect all of your fish and the tank as well. Below is a writeup I did for another posting; hopefully it gives you food for though.
Read up on Brook and Ich. Brooklynella is a parasite that has very similar symptoms to various forms of Ich which includes Marine Velvet are also parasites. Brook is treated with Formalin and Ich is treated with copper. Some of the symptoms are rapid swimming, swimming at the top of the tank and gasping for air, fading color, sometimes spots on the body but not always, sometimes lesions on the body but not always. Brook can take your fish in a matter of days, Ich takes a little longer but it too will kill. If one fish in your tank has either of these parasites then ALL of you fish have the parasite even if they exhibit no symptoms and they have to be treated or eventually they will die. That means setting up a treatment tank with copper in it and giving them Formalin dips. The tank the fish came out of also has the parasite in it and all of the fish have to be removed from the tank for a six week period to stop the life cycle of the parasite. The eggs of the parasite are in the bottom of your tank and can lay there dormant for up to six weeks. When they hatch they have to find a fish host or they will die. After six weeks the tank is safe for new occupants. Get on Google and read and read and read some more about these parasites and how to treat them and do it now before you lose all of your fish. There is NO Quick answer, there is no one day cure or one chemical cure. These parasites are difficult to get rid of so from now on, QT your new arrives for three weeks before you put them into your tank.
