Well I never have thought of it before but I guess a planted tank is a tank that has any live plants in it.
Okay now to the questions:
1) You don't need a filter since the plants do your filtration?
Yes plant do act as a filter however to keep your tank healthily maintained you will have to keep a small bio-load. By this I mean not many fish. So if you would like a larger bio load it is good to have extra filtration. Also filters help to get rid of any floating debris in the water making the water clearer.
2) What substrate do I need? Is it goign to cost me a fortune.
You don't really need any special substrate at all. You can just use plain pea gravel. Especially if you plan on keeping a low tech tank. What is a low tech tank??? It is a planted tank that has minimal lighting, most of the time: no CO2 and is hardly fertilized. From what it sounds like this is what you are going for. However you could just buy a small bag of flourite
and mix it in your gravel.
3) How often would I have to do water changes?
It is generally recommended to do about 10% weekly. Some people like to do about 15% bi-weekly as well.
4) Is a CO2 system necesarry?
As mentioned above not if you plan on a low tech tank. However CO2 is a great addition and you will see healtier and faster growing plants after the addition of CO2.
5) What do you do at night when CO2 goes up and 02 goes down?
That really isn't a problem. there is plenty of O2 left in the water after a full day of photosynthesis for the fish.
Hope this helps!!! Kyle