Originally Posted by BigEd
Sorry but @ 21C you are brewing ale. The fermentation has slowed because it is about finished.
Or he might be brewing a lager much too warm...(too warm for ale too in my opinion).
What yeast are you using? If it is truly a lager yeast, you should be fermenting much much cooler. It will ferment at 21C - quite rapidly in fact - but you will end up with a bunch of off-flavors.