I believe if you brought out the microscope, you would probably die of a heart attack, presuming that all the things you saw were bad, there are going to be millions of organisms in a mature aquarium.
i use them...but not just for parasites and things. there are all sorts of cool critters living in my planted tanks. i do it for fun and out of curiosity. im also a huge biology geek, so...
happy scoping
cheers