Okay so I got 6 Goldfish from Petco just over a week ago and just put them into the tank over the weekend. Anyway, all was fine but one just died last night. The reason I am posting about it is because I want to figure out what happened so that the other 5 don't get sick. They all seem fine and are very active so I don't know. The first few days they were in a large probably few gallon bowl and I even did a half water change when the water got really cloudy. Then I bought them a 20 gallon long tank during the $1/gallon sale so I set that up along with a Tetra Whisper 10-30 gal filter and let it run overnight but not 24 hours to test and put them in around noon between classes. I didn't cycle the tank before I put them in because I didn't realize if you needed to but I did use water conditioner and I just bought Tetra SafeStart to put in although it might be a little late? Anyway, all other fish are healthy and eating. Well, some don't seem to be eating as much while others are pigs. The tank just has one silk plant and hiding thing in there now but I just bought more to add so they're not bored/feel more safe. I bought Tetra variety pellets I think but they didn't like them so I got some Omega One Goldfish small sinking pellets and some flakes so they eat the Omega One pellets now. We plan on adding an air pump and bubble stone or something too, have it bought, just need to install. Also plan on moving them to a pond eventually. They are the feeder goldfish. Anyway, as for symptoms, This one was kind of swimming sideways, not completely on it's side or horizontal or anything but just tilted/leaning a little to one side. That happened for maybe a day or two and then it died. So pretty fast death. At first i thought maybe it was just cause it was tired or current from the filter. Here is a picture of it. Does anything look weird? Was it just a weaker fish or was it sick?