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Thread: Caterpillar Droppings, FC and new growing stain on liner

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    Caterpillar Droppings, FC and new growing stain on liner

    This is my first question here but I have been a long-time reader of the forum and have been able to find answers to almost all of my pool questions here so first, thank you for all of the knowledge shared here. I was unable to find a similar issue so here goes:

    On Friday night, I did my weekly (albeit a few days late) shock and algaecide treatment after thoroughly cleaning out my DE filter and vacuuming up a lot of leaves and "dirt" (which I realized today was caterpillar droppings). On Saturday, I was getting weird reading on my home test kit (probably due to old solutions) so I went to the pool store for testing. FC 9, TC 9, CH 170, CYA 30, TA 120, pH 8.1. Recommendation was to get fresh test solutions, wait for sun to burn off chlorine and lower the pH but otherwise, things were looking fine.

    On Sunday, lowered pH to 7.4 with dry acid, noticed fresh pile of leaves and "dirt," FC was 0 and TC was 3, TA and CH unchanged from Saturday. Added 2# of shock and swim to attempt to get the pool swimmable for the afternoon but it didn't move the FC reading. No time to address further.

    Monday, only had time to try shock (4# turbo shock) and algaecide as I thought perhaps algae was eating FC despite no visible algae. Tuesday morning, FC 0 and TC 5. Both days, all other readings are the same as Sunday and in an acceptable range.

    Tonight, realized that the dirt is caterpillar droppings, which probably explains my FC problem (correct me if I'm wrong please!). So...vacuumed thoroughly, will shock again. However, when I vacuumed, it became apparent that there was a new, off-white stain on the liner. The spots that were covered with debris are fine but for about 75% of the rest of the liner, there is an off-white tinge coating the liner. It doesn't brush off so I don't think it's algae. Is it possible that it's a reaction of some sort? Any idea to what and how to correct it? I've had this pool for 10 years and have never seen anything like this.

    Thank you for any insight you can provide!
    15 x 30 in-ground pool, vinyl liner, 20k, Hayward DE filter, Hayward in-line chlorine feeder, manual vac

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    Mod Squad pooldv's Avatar
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    Re: Caterpillar Droppings, FC and new growing stain on liner

    Welcome to TFP!

    Ok, here is my theory. Given your high PH and TA your pool has developed scale. The acidity of the caterpillar droppings dissolved the scale while it was sitting there for a few days. Does the cleaned off spot feel smooth and next to it feel a little raised up and rough?

    If you enter all your rest results into Poolmath it will calculate your CSI for you. Keep it negative to prevent scale. You can lower it -0.8 and brush as often as you can to dissolve it. Slowly, it would take weeks or month depending on how long it has been forming.

    More here, Pool School - Calcium Scaling
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    Join Date
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    North Easton, MA
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    Re: Caterpillar Droppings, FC and new growing stain on liner

    Thank you - I'll have to get into the pool tonight and feel for the texture to verify but I think you're right.
    15 x 30 in-ground pool, vinyl liner, 20k, Hayward DE filter, Hayward in-line chlorine feeder, manual vac

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