That isn't the filter, it that the tank hasn't cycled. (the nitrite problem any way).
As far as the ph goes, if its <8 then don't worry about it. From what I have heard people say here, the filter that comes with the walmart special is a marineland penguine filter w/o the bio wheel. They have even said you can buy a bio wheel and put it in there. I personally like Aquaclear filters and never use carbon my self. It doesn't last very long, and regular water changes are better anyway.
When you use the Venturi on the Millennium, it creates noise. Lots and lots of noise cuz bubbles are mixing with the impeller. It is not a bad idea, though. It just needs to be quieter. It sounds kinda like when a HOB power filter is starting and it expels the air out of the intake. You can turn off the Venturi. The Whisper has larger cartridges and better flow rates. Millennium has a cartridge half the size of the Whisper and has a flow rate of 85 GPH. The Whisper has a cartridge size Large (look at it and compare with others at the petstore) and has a flow of 105-125 GPH. The Millennium 1000 has a cartridge called Primo. Whisper cartridges (when bought in bulk) are cheaper.
Buy an Aquaclear and just fill it with sponges. Rinse them in the tank water that you syphon out every week. The sponges will last forever and be cheaper in the long run.