The toilet in my condo is one of those modern low volume units.  Any reduction would cause it to stop functioning.

This was a good idea before low volume toilets became common. My dad used to put a couple of bricks in the tank or adjust the float to reduce the amount of water used in each flush. I guess short of going back to out houses, the low volume toilets are the next best thing.

If I displaced any more water in my current toilet tank, there wouldn't be ANY water to flush. My apartment's toilet uses so little water, which is fine with liquid waste, but not all waste is liquid. ADDED: Oh, shoot, I almost forgot: My best friend's mom and sister had a very strange habit. Now, they were very smart ladies, and I thought they were great and all, but they took the "environmentally-friendly" thing to a whole new level: They didn't flush the toilet if they just peed. . . . ew. My best friend said that they said that "it didn't smell" or anything. He didn't care much for their little ritual, either. Sorry, I'm all for trying to be environmentally-friendly and all, but this one's a step or two too far for me.

Modern toilet tanks are already so low flow that any further restrictions would render the unit inoperative.