what could this be???

[Frank]

Hi everyone and poolguy, just a quick query. 

When I take the cover off our pool in the mornings, the walls of the liner are clean, the water is clear, the chlore and ph levels are normal, yet when I come to put the cover on at night ( we have guests) I have noticed dark patches have appeared on the pool walls near the bottom.  I am at a loss as to what could be causing this, anyone any ideas??? Do I need to introduce fresh water??? I actively 'winterised' over winter and kept a close check on water quality, but it seems to have started happening this last couple of days, the temp has risen from 14 to 21 degs since sunday,   I could understand  there being a problem if the levels for chlore and ph were wrong, but they are not, any advice/suggestions greatly appreciated.  Many thanks in advance

Jetlag

[Poolguy]

I am afraid you will have to be a bit more specific about the problem.

'Dark partches' .... not a lot of clue what that could be.

Perhaps if you could email me some pics it might get a bit clearer.

Also could you post the results of your tests and if you have vacuumed.

Andrew

 

[Gemini_man]

two possible causes from my experience:-  it could be due to moisture on the underside of the liner or if you are running an electrolysis device it could be due to excessive use in which case switch it off. other than that sorry no idea!