I am having some questions about my fish tank, please help?

so i got a fishtank (20gal) and its got a heater, filter, bubbler, thermometer, everything u need. i put pH regulator in it the day before i put the fish in so my ph was at a dead neutral, and i put tap water conditioner in the water prior to putting the fish in, and the temp is at 76degrees for my freshwater tank. i have 3 fish, an angel fish, and a silver tipped shark, and idk the name of the last one but it was all black. i found it dead this morning. this is my 2nd day having the tank and i got it last night and the next morning it was dead. idk what happened, and also i want to know why the 2 other fish wont eat the flakes i put in the tank. also i was wondering, what if one fish eats all the other food that was for the other fish?