Objective
This guide helps you diagnose mysterious dark stains on the bottom of your vinyl liner pool and provides the specific chemical steps needed to treat soil fungus (often called "Vinyl Plague").
The Truth About Vinyl Plague
If you own a vinyl liner pool and notice a dark, spreading stain that refuses to brush off, don't panic. While it is commonly misdiagnosed as a metal stain or black algae, it is often a condition known as "Vinyl Plague."
Because vinyl is a semi-permeable membrane, a specific type of soil fungus thriving in the damp, dark earth beneath your pool can actually grow its microscopic filaments through the liner to reach the water. Understanding that this is a biological issue originating from outside the pool changes our entire approach to treating it.
Prerequisites
Verify your pool surface is a vinyl liner.
Have a standard 3-inch chlorine tablet (Trichlor) on hand for diagnostic testing.
Step-by-Step Instructions: Diagnosing and Treating the Fungus
1. Perform the Chlorine Diagnostic Test
Because Vinyl Plague looks identical to metal staining, we must test it first. Place a single chlorine tablet directly on a small section of the dark stain and leave it for 5 to 10 minutes.
If the stain does not change color, you likely have a metal stain. [Insert internal link to related article: Identifying and Removing Pool Surface Stains]
If the stain lightens or disappears under the tablet, you are dealing with Vinyl Plague.
2. Execute the Initial Fungal Treatment
Because the fungus originates in the ground, completely curing it permanently requires removing the liner and treating the soil. However, you can successfully control and suppress the visible bloom in the water.
Turn your System Power to the ON position.
Add a heavy, targeted dose of a strong algaecide, such as Swamp Treat™, directly to the water.
Immediately superchlorinate the pool using Trichlor-s-triazinetrione (granular shock or tabs) to aggressively drive the fungus out of the liner's surface layers.
3. Establish a Preventative Regimen
To prevent the fungus from pushing back through the liner, you must maintain a slightly higher baseline sanitizer level and introduce a preventative barrier.
Add No Mor Problems® to the pool weekly during your regular chlorination routine to act as a preventative algaecide and fungicidal barrier.
Troubleshooting Note: Protecting Your Liner
When superchlorinating, be cautious not to let undissolved granular shock sit directly on the vinyl for extended periods, as this can bleach the liner's printed pattern. Always pre-dissolve granular shock in a bucket of water before pouring it into the pool.