I have a light color tub and had the brown line across the tub (happened after the first night), read a thread about Spa Perfect (enzymes) with Chas and Stuart saying positive things about it.
I went out and bought a liter bottle and after the first application, the ring disappeared. It even seemed to help with the foaming somewhat. I just put the second application in this morning.
I found a seller on ebay who was selling a 3 liter bottle for less than what I payed for the 1 liter, so I picked up 2. I figured if it did that with one application, it should be a weekly addition to keep the ring and foam down.
Might be worth a try!