Cloudy water & dying fish & snails

Question
hey,   my tank has been cloudy for a while maybe just a week or so, but when it went cloudy, my fish started to die.  so far one or my neons died, my male guppy died, my 2 platies died and 1 long finned danio.  also, my one tetra has a white cloudy thing (fungus?) on it, so should i use melafix or what.  
my tank is a 20 gallon tank, it is heated, and has a filter.  the other fish i have in it are: peacock eel, 6 tetras, 1 platy, 2 guppies, 1 dwarf frog, and 2 danios now.

how do i get rid of snails??  i keep getting stupid snails from my plant, but i dont want to get rid of my plant.  so how do i get rid of snails??

Answer
Ok this problem has happened to one of my tanks before, and my nitrites and ammonia were high which led to dead fish. If you haven't checked ammonia and nitrite i suggest you do it. To treat the fungus i would use "Jungles cloud clear" and that should cure it. To get rid of snailsi would get a horseface loach because they eat snails. To fix the cloudy water use "Jungles Cloud Clear"