if you are just giving your plants water with no nutrients then yes it is normal. Since you are not supplying them with nutrients or food, they are using their stored food / energy supply in the leaves. Thats why they are turning yellow. If you are flushing your plants than this is fine. if not, get some nutrients/food to them.
just fyi.... plants can also get some yellowing of the leaves when switching from grow to bloom nutes due to the lower nitrogen content in bloom nutes. Not a big deal but if its bothering you then just give them some Nitrogen...