yes i am new... and i ask for some insight......

1st thing is i have a 2gal resivor with three plants one is starting to trun slightly yellow on one leaf.. no burning curling or nothing i am seeing that it is getting lighter in color.thats it.... the water is the first batch i mixed up about week and half ago 11days the ph is around 6.0------can this yellowing be a first sign of lack of Nitrogen..

how often should i change the water...once a week or twice ......