No Problem, Earth.
I acquired a 20oz. bottle shortly after I did this. See, when I hobbled to the store to buy band-aids, I felt like a refreshing 7-Up would hit the spot. The bottle has been hanging around my kitchen sink, waiting to be recycled. And, since it has taken me forever to find a place to take my paper recycling, I knew this bottle was going to be around my sink for eternity. But then, I read about a little somethin'-somethin' you can do with bottles.
Operating solely on memory, I dunked the bottle into the tank of the toilet and filled it completely with water. I set it up in the tank and voila! my toilet is now using less water due to the displacement of the bottle.
I should have found and read some instructions on this before actually doing it. All of the instructions I discovered to link to say to put some sand or rocks into the bottle before filling it with water to make it stable in the tank. Which, is a good idea. Once I flushed the toilet and noticed it was running longer than normal. Sure enough, the bottle had tipped and was preventing the toilet from doing it's thing. (That's why it's now set in the corner of the tank.)
Give it a try. It's easy, free, and you don't even have to find a place to recycle your one lone plastic bottle.