There is white stuff growing on my fish . what is it?!?

well i got 3 fish on my b-day i had my fish tank set up and set the Temperature and de-polluted the water with aqua-safe from tetra Aqua water conditioner and fed them tetra aqua tropical fish flakes and makes them urinate and do less feces and 1 week later they are growing white stuff on their gills and the food will grow a clear- whitish layer over it i changed it 50% but i was recommended 100% and im not sure if it will help and i had 2 of them die. Oh and im a new fish owner so im clueless about anything !!