TDS is total dissolved solids.
The test strips are very inaccurate in most cases.
Where does the ph need to be before using the controller? I was under the impression that the controller simply measured the ph change associated with adding co2 (which acidifies water, thus lowering ph), and shuts the co2 off when the ph has dropped to a certain point, indicating the proper amount of dissolved co2 in the water. Don't fret about the ph changes due to the addition of co2. These don't really affect fauna. Changes in the TDS, which has nothing to do with co2, are really what affect fish.
Fighting with your ph is a pain and a losing battle in most cases. There are only a few ways that ph could have been lowered (peat, some driftwood, ro water, buffers, co2). Water changes are generally a good thing, as they help keep the water clean. Read this article to get a better understanding of water chemistry in our aquariums. http://www.drhelm.com/aquarium/chemistry.html
Running an airstone has its advantages as well. I don't use them, but lots of folks do, with no ill effect. As long as you don't run it when co2 is running, you'll be fine. If you did run it along with co2, you'd simply degas the co2 you are adding.