Fist let me provide some basic information :
Pool - Above Ground - 24ft round Steal pool - approx. 13,000 gals.
Frog inline Chlorination system (Mineral pack + Chlorine pack)
Filtration is Sand filter with 1 horse pump
I've had the pool now for about 7 seasons. The first liner (yes I said first liner), started developing brown staining after the first or second season and progressively got worse. I spoke with the pool company and there best guess was ground mold, which I was told once it takes root in the liner there is nothing I could do. So about 2 seasons ago I replaced the liner, and had them install a plastic matting under the liner to prevent this from happening again. Well I've now stated seeing this brown staining again. Should be noted the water is and has been crystal clear. I asked the pool store if the Frogs mineral pack could be causing it and was told no.
A few additional pieces of information which may help is I have wedding cake steps in the pool. To hold them down the steps have 3 bladders which need to be filled, the instructions call for sand but I chose to use rocks / gravel (bought a couple of bags at the home depot). I have test strips but to be honest haven't used them much. I have had the water tested at the pool store a couple of weeks ago and was told I needed Ph Up and Alkalinity UP which I added. Had the water tested again today, and was told my phosphorus was a little high so I'll be adding the appropriate chemical for that tomorrow. Here are the numbers provided by the pool store:
Free Chlorine: 0.0 ppm (I shock every weekend, and have 2lbs of shock waiting to go in but was told by the pool store not to shock it for 2 days after applying the Phosphorus chemicals
Total Chlorine: 0.2 ppm ( I also bought a new Frog backpack to replace the one that is almost empty, but again was told to wait on installing for 2 days).
Combined Chlorine: 0.2 ppm
Hardness: OVR247 ppm
Alkalinity: 35 ppm
Cyanuric Acid: 38 ppm
Copper: 0.1 ppm
Iron: 0.1 ppm
Sorry for the long post. I asked them if the staining could be Metals, but they tell me my metals are low. I guess I'm hoping someone will tell me its not ground mold, but something easily correctable so that I don't end up having to purchase a third liner with in the next couple of years.