Less! Although you may hear many English speakers use the word lesser, it is actually improper grammar, and not a real word. In some situations you would use the word fewer, but that is only when the thing that is decreasing can be counted and is plural. For example, you could say "I have fewer plates than him," or "There are now fewer hats," but you would say "I have less water," or "I have less time." Hope that helps!