there are all kinds of things in tap water that WILL NOT be removed by a simple filter. In example,...cities all across the usa add phosphates into the water at the treatment plant to discourage the pipes from corroding. There are also silicates, and not to mention that most of us have copper pipes of some sorts in our homes. Phosphates and silicates promote hair algae growth...which you dont want. Copper kills corals. And then there are the solids in the water that are undissolved and will also fuel the algae growth. As far as what the solids are composed of depends on what your mind can consider has been dumped in the rivers, lakes, streams, toilets , sinks, all around your area and has made it to the treatment plant and since they dont use RO\ DI....lol...you end up with a lot of lil nasty things in the water that you cant see but WILL give you problems in the long run....dont use tap water.
Good Luck