The O2 system is a quality prespray and extraction rinse combo that were developed to work as a great system for residential carpets. The developed a booster called their hot sauce modifier that boosts the pH and adds enzymes to quadruple the potency. Both boosted and straight out of the jug, the system is a residue free clean and the dilution ratios are really make the system a money saver.
The basic Prespray is an oxygenated citrus based cleaning solution. utilizing Sodium Borate as the oxidizer, and a speceal blend of surfactantss and citrus solvents that work very well to emuslfiy and brighten without the usually harmful effects of an oxidizer.
The rinse adds a touch more oxidizer as well as a drying agent and neutralizing agents that remove all of the prespray, while leaving no residue of its own.
The pH is below 10.0 pH and will not harm the stain resistance of the carpet.
Judson Labs produces this product and it is very popular amongst users on this board.
http://www.truckmountdepot.com/Chemicals.html
In your particular situation of a water stain, generally the brownish residue and mineral deposits left behind are organic in nature. Thus an oxidizing product is going to help neutralize and break down those compounds. The o2 system is good choice for this application. As far as encap products go. Someone mentioned
Releasit DS, but in this case i woudl recommond Rick Gelina's Encap
Hydrox. Its an oxygen based encap that crystalize just as well as the rest of the
releasit line. But was developed specifically to tackle water marks and organic stains, soiling while providing a low moisture solution to eliminate wicking.
However, you mentioned that the carpet has an attached rubber back. This means it could be either C/A which is a great product and the above solutions would work. It could be an off brand of carpet from the box stores, in which would worry me. But worse case : If its a black backing that is kinda of deterorating, gummy, or powdery, it could be older viking back carpet. In that case it may not be a water mark problem alone. That carpet is known to break down with time and moisture could bring those residues to the surface creating a terrible stain that can be very difficult to remove with out causing further problems.
if it is a deterorating underside rubber pad, I would use the encap product solution to minmize further damage to the floor.