LOL! If your water parameters are good, you do water changes, and you've got an air stone/waterfall, you don't need to be worried about oxygen. You'll just know all is well. I think you'll find that most of us do not use tests such as these...
Funny, but I don't ever worry or even think about oxygen...I know I'm well aerated, so I just don't have to think about stuff like that.
Save your pennies for a water quality test kit, rdk, The other kinda stuff is just unecessary or for perhaps for pond keepers of fish that have prize winning koi. I've really not come across someone who uses something like an oxygen test....