I wouldn't worry about nitrates to much. Many of us with planted aquariums actually have to dose nitrates because vigorous plant growth depletes it quickly. As a infrequent top up, it should pose few problems.
Depending on how big the pond is you can spray the water into the pond as the aeration helps remove the chlorine. If it's a small pond you can leave some water in a bucket overnight and the chlorine will gas of of just top up a little a day.