Yes, it's something to worry about. How long has the aquarium been operating? It needs to go through a nitrogen cycle, which can take up to six weeks. Basically, it needs to build up enough good bacteria to convert the waste products (mostly toxic ammonia) into plant food. That's a pretty bad explanation; I like to think of it as growing good cooties, which live on the plants, decorations, gravel, etc., but not in the just the water. Putting old water into a new aquarium will not help the cootie cycle, but putting in gravel and stuff from an established tank will help.
Here are a few other thoughts. If you just got the angelfish, they may have already been sick or infested with a parasites. The most probable cause, however, is shock. The conditions in your aquarium may be quite different from those in the pet shop or the fish farm that sold the fish to the pet shop. Angelfish can thrive in a variety of conditions, but rapid changes will put them into shock, which in turn compromises their immune system. Using the drip acclimation system will help mitigate the shock. Also make sure the water temperature is 80 or 82.
This post is getting too long, so I'll try to write something about the drip method in a different post.