The reason your tank is a sewer is typically because you overfeed and do not do enough water changes. My high nitrate, high phosphate tanks are pristine.
While this is certainly true, it is a false assumption that simply having a high NO3/PO4 tank equates to the habitat contamination. There are many substances that contribute to contamination and those substances that are truly responsible for contamination, such as organic waste are easily controlled via water changes.
Bizarre responses, I'm guessing you didn't read for context