Fish - Aquarium

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For a long period of time my colleague was pursuing me to have an aquarium in my house. My brother-in-law had one and as he lost interest in it he gave it to me with a Shark Fish. As it was alone in the aquarium, I added three Molis Fish, white, black and orange. Later I added two Gold Fish. I observed that the shark was very active in movement, Molis were good and the Gold Fishes were slow moving. After a couple of days I saw there were 25 to 30 tiny fishes, must be Molis. I transferred it to a separate plastic bowl. After about two days I observed that the Molis were poking the Gold Fishes, next day I saw one Gold Fish dead. I moved the other surviving gold Fish to a separate plastic bowl. It was dull for some time. Now the shark fish became slow moving and after a couple of days, it was under stress and I thought it is going to die, then it also died. The Gold fish stopped eating food for two days and it also died. Then I saw orange Molis always lying in the bottom and one day I saw it dead. Then the white became very over active and after a couple of days, it also died. The Black Molis was lonely and it also died very next day. I used to feed the fishes with Fish Food powdered as well as whole grain and was treplacing the water (borewell water) every alternate day with fresh water of about 50%. I used to feed the tiny fishes with powdered Fish Food and was changing the water (borewell water) every alternate day. After some time I saw every fish dying day by day and all of them dead. What's wrong with my feeding / maintaining? Why all the fishes died. Please, can anybody educate me on this? I am planning to buy 8 - 10 Molis or any other Fish, and I don't want this to be repeated. Please help me.