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Root rot is a condition more common on indoor plants with improper drainage and hydroponic systems caused by low oxygen mostly caused by temperature since low temperatures lower dissolved  oxygen while high temperatures cause low levels, so keep temperature between 65-72F. This is often caused by Pythiaceae water mould including the genus pythium which was once classified as a fungus since if responds to many treatments similarly. It is part of the parasitic oomycete class that's considered microorganisms. This is typically reversible and can be prevented easily by creating an environment is simply can not survive in. This includes not letting your roots ever just sit in water.