I have been reading the "Ecology of the planted Aquarium by D walstad" and her books says that mulm is good in planted tank because the bacteria soon break it down into useful nutrients for the plants. It also says that the mulm will soon because humus(humic acid) material after the bacteria process is over after some time. I know from horticulture that humus is good in ground plants and help in root growth. If all this is true why do people vacuum out the mulm in their tanks? Shouldn't it be mix in with the soil to add back all the elements needed by the plants? I'm still reading through the book but I don't recall reading anything about too much being bad, unless I missed it. Can someone help me understand this process a bit more?