My fish might be dieing? Help!!!?

Yesterday I went to petsmart and got feeder minnows, for my red and tiger Oscar, I woke up the next morning and half of them were dead, and had white spots on there tails, I thought it was ick, but how could ick kill fish overnight, and when I bought them they were perfect little minnows. Well when I went to feed my Oscar it's regular pellet food my tiger Oscar was rubbing it's body on the rocks and would attemp to swim to the top but he didn't have enough power so he would fall rapidly to the bottom of the tank. He lays sideways on the gravel. I know they do that and are known for it but he hasn't done that since I got him a month ago. The feeder tank was fine. It had a bubbler water conditioner and but no filter it had 15 minnows in it. It is a 2 gallon tank. Is my fish dieing?