Ok you DON'T need that Ph stuff in the bottle. Your Ph is probably fine, so no need to mess with it.
DO NOT buy any more fish untill you know exactly what you are doing, or you're just going to kill more fish.
Go ahead and read the links posted on the previos page, they'll help you understand how to make the tank safe for fish.
A filter cleans the water to a degree, but you STILL need to change the water out at least once a month. Theres no getting around this. Filters don't clean out diseases, they only help take out chunks of stuff and chemicals in the water, like Ammonia, etc.
Unless you're on a well, your water has Chlorine in it, you know, the stuff that makes it tase bad. This is VERY bad for fish. To remove this chlorine, you have to buy a bottle of dechlorinator. Some brands are AquaSafe (comes in a yellow bottle), Prime (white round bottle with a red label), or Stress Coat (yellow label). Any of these are fine. Add however much it says to on the bottle to the water. Don't worry if you add a little extra.
From what I gather, you want to set up an Amazon themed Pirahhna tank. Do some research on Pirahnnas before getting any more.
If you have any more questions, go ahead and ask, but please make your posts readable :duh: