My fish are dying ....what am i doing wrong?

I bought 3 fish and at first i used jug water and they were fine but they made the tank dirty in a two days then i cleaned it out and then i used tap water and i deluted it ....i can't spell but kno what i mean then a few days later the fish died and then i cleaned out the tank again and then a few days later my other fish died.....now i have one left.... when they die they get a weird fungus on them and the food at the bottom of the tank looks fungus like too....am i feeding them the wrong food? whats going on?