But, if you shock, the stains may come back. It is a vicious cycle. But, if you have overnight chlorine loss then you have something in your water that is brewing and you have to kill it. That is why once you get your metal/stain issues dealt with, it is critical that you always keep your chlorine in range for your CYA level (based on the Best Guess Chlorine Chart in my signature) so that algae can't get started.

If you do have algae brewing, you will have to kill it and then deal with the metals and stain issues again.