Tips on how to prolong the life of koi and goldfish?

I'm starting to like them. They are already learning to feed from my hand and beg for food. But I'm scared that they might not live long. I feed them flakes twice a day.I avoid overfeeding them. I change the water once a week. I have an airpump and filter and i remove chlorine and metals from the water. Are there still tips I can use to make them healthier and live longer? And, is it true that they can eat bits of lettuce & watermelon?Thanks!