Algae Growing Under Pool Liner
by Pat Garrett
Wet/damp spots, algae under liner
I need to replace my above ground liner. (21 foot round, frog system, very little chlorine). It was leaking down more and more over a few months last summer and lost half the water last winter. When I pulled up the old liner I found many spots where it looks like it was leaking. (See the photo) is that what I am seeing?
If so my 25 year liner appears to have failed across every panel.
My guess, and I have seen this many times, is that the bottom of the pool was full of pin holes. Pin holes leak slowly, like you describe. The most common cause of pin holes are automatic pool cleaners. If the cleaner runs too often it is possible for it to wear holes in the liner. If that sounds like your situation just reduce the amount of time the cleaner is in use, and be sure it is always in motion, never cleaning the same spot for hours at a time.
In most cases where the algae is in the sand like that there is plastic under the sand. With no plastic the water sinks into the ground and the sand remains dry, not producing algae. There are many times when plastic is needed, so it's not a bad thing, just something to be aware of.
Before installing the new liner you should remove all the algae and probably add a little more fresh sand.
Some important things to know before going into any more detail would be how old the liner is, what brand and what thickness. Also helpful would be the type of cleaner used, if any.