Ok fishgirl, this is how it goes. When you get a new tank it needs to be run in for a while for the bacteria that are required to make your water safe for fish to grow. this can take up to a month. Some people put in one or two fish to help this process, some people use ammonia. This is called cycling. Do a search on google for specifics on cycling.
Not going through cycling and just adding fish will basically turn the water into fish poison. Some might live, but most will likely die.
The stuff you add to your water removes harmful chemicals such as chlorine, but does not elp with cycling.
Next, there is the size of your tank, and the fish you have in it. Oscars are far too large. When they grow up, they will not be able to turn in that tank. All sharks also get too big for that tank. And the pirahna type fish you mention could be mots of different fish... you need to find out names.
Next, the filter. This is essential to your fish. Leave this off and your fish stand less of a chance of living. This is where much of the bacteria grown during cycling will live, and it will help to make your water safe for the fish.
You absolutely have to research what you are buying before going out and just getting what you think looks good. the impulsive approach will lead to incompatibility and more, resulting in the death of your fish.
Please, for the sake of your fish, return them to the fish store until you have spent just a little bit of time figuring out what you need to keep them healthy. Otherwise your fish will just keep dying, you will be wasting money, and will more than likely grow to hate the hobby before you go to enjoy it.
