Hi, I am relatively new to keeping an aquarium and I am really scared that I am going to kill all my fish. I got my tank about 2-3 weeks ago and it is fully cycled. I got the water tested and got my first fish a peacock gudgeon. I have had him for about a week and a half now and today he was fine swimming around and everything but when I got home he is dead. Do you think it is something i did wrong or was something just wrong with the fish?