Color Fading Below the Water Line
There can be many contributing factors that lead to the fading of your liner. All of those factors can be grouped under the heading of chemical attack, however the leading cause is simply over chlorinating. Just as excessive use of bleach will fade your clothes, over chlorinating of your pool water will greatly accelerate the fading of your liner.
From the vinyl's standpoint, any chlorine level above 3 PPM will accelerate the fading process. The use of a chlorine based sanitizing system is going to bleach your liner. There is no way around this fact. The higher the active chlorine level, the quicker the fading will occur.
Be especially careful when shocking, closing or opening your pool. It is critical that you circulate the water for a minimum of 72 hours after any of these procedures. The average shock treatment is going to bring the chlorine level of your water to at least 25 PPM and as high as 50 PPM.
The specific gravity of the chlorine is higher (weighs more) than that of the water. It is therefore critical that you circulate your water long enough to insure that the chlorine will not settle out of the water and concentrate in the deepest part of the pool.
It is also important that you do not cover your pool for at least 24 hours after one of these treatments. The covering of the pool will greatly restrict the chlorine's ability to dissipate, thereby greatly increasing the likelihood of damage.
Our experience has shown that of all the sanitizing systems, Trichloroisocyanuric acid has the greatest potential to bleach a vinyl liner. Alkaline sanitizers (Hydochlorite) are much more vinyl friendly and just as effective. No matter what system you use, always use the minimum amount of chemical that will get the job done.
Remember: less is best when adding chemicals to your vinyl lined pool.