What can i do for my fish?? HELP!?

A week ago a got two fish from the carnival. One of them died because the other attacked it so at the moment it's just the one fish bubbles in the tank. Anyway the tank water keeps going cloudy like overnight and it just gets worse the longer you leave it . so when i got home today i cleaned the tank and put a small amount of food in because i had just got rid of all the food in the tank previously. Anyway while my mum was cooking tea i noticed he kept on putting his top fin down for five secs then up then down and so on. Then as he did it with his top fin he began to do a similar thing with his tail. Now he's just sat at the bottom of the tank and when he eats he keeps on spitting out the flakes. So i too out the remainder f the flakes and put a tiny amount of the non-flake food he was eating before in but i'm scared he might die. I'm getting a new tank in a a few days of my uncle so he won't just have to stay in that carnival tank any longer. What have a done wrong?? How can i help so Bubbles won't die? Please help!!!!