i read (on this site) that nitrate levels may be the cause for floating fish and that reducing the n02 levels could help with SBD. i wanted to know if it is possible to change the n02 levels, and how i might go about doing it.

You can lower nitrate levels by (the obvious one) changing the water more frequently. Also by having live plants, nitrate levels will decrease as well. There are more expensive options, such as using a protien skimmer (though not sure many people use it on goldfish tanks).

N02 is nitrite and N03 is nitrate; both bad for your fish but I believe goldfish are more tolerant of nitrate than nitrite. In the nitrogen cycle your fish give off ammonia through their gills and in their excrement. Bacteria in your filter convert the toxic ammonia to toxic nitrite and another type of bacteria convert the toxic nitrite to a less toxic nitrate. Plants such as algae consume nitrate but may not utilize all of it. There are chemicals that can be added to the water that will "tie up" the ammonia and nitrite until the filter is cycled (can take 6 weeks or longer) or you can take the popular but labor intensive route of making frequent water changes to keep the ammonia, nitrite and nitrate below toxic levels. Of course, so that you will know where you are, you will need water test kits for ammonia, nitrite and nitrate. You can ease the situation by giving the fish more room (larger tank and/or fewer fish) and feeding less. Also, maintaining the proper pH of the water can help since I understand the nitrogen based biproducts to be more toxic in acid water (pH below 7.0). I've also been told and read that salt in the tank water will reduce the nitrite problem. You can use aquarium salt (I use ice cream rock salt) at the rate of 1 tablespoon per 5 gallons of water to give you a 0.1% salt concentration. You can read on the nitrogen cycle in tanks till you are blue in the face and very confused. Good luck!
Milo