AquaMan: I wish I could tell you a specific reason but I can not. The fact that you had it in a QT tank is a great plus for you and your sense in doing a QT tank. The fact that it lived 3 weeks gives me the insight that it probably was not healthy to start and the fact that it was alone it gave its problems time to over take it. I have bought a few fish that start out well and in a matter of weeks they suddenly get bad and pass. Some of this is due to the collection methods of the fish collectors and some due to the fact that the poor fish is just moved around to much and to quickly before you buy it. To clarify it for you go ahead and check the water to make sure it tests OK, and if it does I would leave the tank running for now. Before you buy another fish of any expense tho I would get a cheap Damsel or 2 and try it in the tank for 3 to 4 weeks and see if it/they do OK. Wish I could be more help but this is best I can do for now.