I have a small yellow plastic float that can be moved/slid up and down the flapper chain. What this does is allow you to (approximately) set how long the flapper stays open because the float will hold the flapper up for a longer (or shorter) period of time.
This is what the float looks like http://www.amazon.com/Korky-2004BP-Plus-Adjustable-Flapper/dp/B0000DAZM8/ref=sr_1_3_m?s=kitchen-bath&ie=UTF8&qid=1384984760&sr=1-3
HOWEVER ----- this flapper and float that I have pictured in this link WILL NOT FIT THE 2021. It appears to me that AMAZON no longer carries a 2021BP with the float available on it so I'me not sure, but you could probably make a float similar with a small square or round piece of styrofoam and put a small hole in the middle then push the flapper chain through the hole?