My fish tank got dirty out of three days or so?

I recently purchased a Tetra 1 Gallon Hexagon fish tank from WalMart with 4 small fish,the WalMart pet-area worker recommended those fish for such a small tank. I bought the AquaSafe and filters but it seems its not doing anything. My younger sister feeds the fish 2-3 times a day (after all its her graduation present for passing kindergarden) But I have no clue what to do, do I start all over and place them in little bags like the movie Nemo and personally clean the tank? We're just a first time fish owning individuals :(