top off vs. water change

Hi all, don't get me wrong here as I do my water changes every two weeks. Just a wierd question. Why change water 15% every two weeks or a month when I add a gallon of water every other day. Which equals my 15 gallons I change every two weeks? I have fish, inverts and corals and I do add suppliments. I'm not trying to get out of water changes, just a curious stupid question that popped in my head. Jeff

 

Adding topoff water doesn't remove nutrients that build up over time....waterchanges do.

 

Water changes as inwall75 pointed out dillutes the DOM, and excess nutrients, and it also add trace minerals to your tank that become exhausted through efficient skimming as well as absorbed by fish, inverts and coral etc.! So because of this small water changes weekly are very beneficial IMO!!!!!! The water change is not to be compared or related to top off of fresh water due to evaporation...Totally different IMO

 

pretty much toping off is keeping the salt content of the water at the proper levels. water changes removes nitrates and etc...and helps keep the water clean.

 

Just like salt, DOM's don't evaporate, hence the need for water changes. (just to put it into different terms)