why do we need to do water changes if aquarium plants take in nitrates

[murad]

i do my weekly water changes with a gravel vac but just curious on why we need to do water changes. after a tank is cycled ammonia and nitrite are 0 which leves nitrate for plants

 

[eco23]

Google "Old Tank Syndrome". Water changes are about a lot more than nitrAtes and cleanliness. TDS (total dissolved solids) build up is a big issue as well as the need to restore nutrients and also buffers which hold ph steady.