Hello, I am new to fish keeping, and this was my first fish. I bought a betta fish from petsmart two weeks ago. I bought one that looked very colorful and active. I put him in a 5 gallon tank, with a heater and a filter. I treated my tap water with declorinater and chemical remover first, and also used tetra safestart to help cycle the tank before adding my fish. I fed him 3 floating betta pellets twice a day as the directions stated. I had two silk plants and a small house in the tank, and natural colored gravel. I washed everything before adding it to the tank. The fish (Dudley) seemed happy and active for two weeks, then I noticed his color starting to fade and he was becoming lethargic. For the last few days, he would not eat at all. He died yesterday, and I don't know why. I really tried to take care of him and thought I was doing everything right. Can someone give me some advice as to what I could do better so I can keep healthy and happy betta fish? Thank you!