My tap water has fairly high nitrates already so I don't worry about starving my plants with water changes. I actually think they benefit from them because the new water isn't depleted of trace minerals and nutrients yet. I change the water in my planted tanks at the same interval as my fish only tanks and my plants seem to be thriving. I don't have an exactly set schedule, but I change 30% to 50% of the water in each of my tanks every 7 to 10 days. I did change it more in the past because I had high trouble with high nitrates, but in my planted tanks, the levels never go above about 10ppm anymore. Some people who are very regemented about their fertilizing probably do a lot less changes than I do to keep the tank more stable(and use less ferts,) but the only things I really add to my tanks are fish food, kent freshwater plant, and co2, and I'm happy with my results.