Can someone help me with my goldfish? I know I am doing everything rig

[Kutay]

Hi. For the past two months, I have been struggling to keep my goldfish alive. At first, I had four goldfish (2 gold, 2 gray). In about a week my brother spilled the fish food and one gray one died. A week later, the other gray one died. Then we got another gray one. This time one gold one and the gray one died. This was because (according to Petco) because the water was dirty since I pre-soaked the flakes which dirtied the water. After I started sprinkling the flakes, we got another gray goldfish. This one died later because it didn't have enough oxygen. So we got a powerhead and I got a more expensive goldfish (thinking I would have no more problems since I fixed them all). First all the fish were stressed because the powerhead was very powerful so I moved it to the way it is now and they got much better. However later the expensive goldfish died and now I am left with 2 gold goldfish (one of whom was ironically with me from the beginning). Please look at my tank at http://www.youtube.com/watch?v=wYVdKeo3R2c. Also I should mention every time I do a water change I add 1ml of Stress Zyme and 1ml of Stress Coat, and I leave the water stand for one night before I add it. I sprinkle a small pinch of flakes and I turn off the powerhead and filter when I do this. I am thinking of starting another freshwater tank with aggressive fish but how can i do that if I cant even keep goldfish alive! Please help me at least keep these two fish in good condition. Thank you in advance.

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