I don't think water from an established tank is a good idea. That water can contain a build-up of stuff that isn't desirable. And, doing regular water changes is a good idea, so starting with "used" water just means you will be changing the water parameters more than usual when you do your first water change.
If you use Seachem Prime, or an equivalent product, every time you add more than 10-15% tap water, you will be neutralizing the chlorine and ammonia that is in lots of tap water, and which is the most harmful stuff in tap water. Ask for, or look for the water quality report for your water company to see what else is in the local water. There are regulations limiting how much of most potentially harmful stuff can be in the water.
If your tap water is extremely hard, you may want to mix distilled water with it, to lower the hardness. Most plants do better in water that isn't extremely hard.