If the brown stain won't brush away...

[docsobeck]

i think i may have posted about this a few years ago, but i can't find the thread, and i don't remember what the responses said.

i seem to remember once upon a time reading a comment about "algae under the liner". is there really such a thing? my water is clear, chemistry is good (CYA 30, FC usually between 2 and 2.5, no CC, ph 7.5, alkalinity good). but when the FC sneaks down in the 1 range, which happens as the day wears on when i don't have the solar cover on, brown stains appear in various spots on the liner. last time i had these, it was a fairly small area, but this year it covers a substantial portion of the shallow end. not dark brown and visually obvious, but just a subtle shading.

since it corresponds to lowered levels of FC, i figure it's probably organic. but it simply will not brush away at all. not even the tiniest little bit. nor does it feel gritty, slimy, or anything else different from the spots that don't have it. it seems to me that if it were some sort of algae growth, (1) i'd be able to dislodge it with vigorous scrubbing, and (2) it wouldn't keep coming back in EXACTLY the same spots, which it does. i'm stumped.

it's not a huge deal: it's just looks.... inelegant. i expect more from myself and my pool! whaddya think?

[dmanb2b]

with CYA of 30, you should not be letting your FC drop below 3ppm, ever. To find out if the stain are organic, you can try holding a trichlor tab in the area for a few minutes, if it lightens, you have your answer. If you suspect it is metals, rub the area with a vit C tablet and see if it improves. Lastly you should perform an overnight FC loss test to see if you need to shock. What is your CC level?

[let_her_flicker]

You can also try Mr. Clean Magic Eraser.

[aylad]

Here's your other thread:
http://www.poolforum.com/pf2/showthread.php?t=1546&highlight=algae

Janet

[docsobeck]

thanks janet, but that's not the one i was thinking of. that was a spring stain, this one is recurring. i guess i imagined it.

so let's say that it is organic. why/how would it be impossible to brush/scrape up, and why/how would it keep reappearing in the same spot? that's the part i can't wrap my mind around.

and if it's not organic, why/how would it respond to higher levels of FC?

BTW, i have 0 CC, and when i shock the stain goes away but then comes back when the FC gets back to lower levels.

[waterbear]

There is fungus from the soil that can grow UNDER your liner. Not much that can be done about it until you change the liner. Chlorine and fungicides are the usual treatment of the soil at liner change time. Some people have reported success by applying a fungicide (often copper based) to the soil around the liner and soaking it into the ground but YMMV.
What color are the stains? The ones from under the liner usually look blackish.
Also, this type of fungus is not really affected by chlorine levels in the pool.

If yours only appears when the FC drops it sounds like common algae, especially if it is brownish. As to why it appears in the same spots, there is a good probability that they are rougher than the rest of the liner (perhaps from the brushing to try and removed the stains). Algae usually appears first on rougher sections of a pool where it can attach itself.

[docsobeck]

i agree that it must be an algae issue, but i swear there is no roughness, pits, divots, or anything at all different about the places the stain appears. very strange.

it's a light brown color, not yellowish at all, but not dark and really noticeable. it's easier to see when the sun is bright; when it goes behind a cloud, it's very difficult to see. guess i'll just have to be more vigilant about keeping my FC level up.

[waterbear]

If you suspect it is metals, rub the area with a vit C tablet and see if it improves.

If a stain is metal you do not need to RUB with a vitamin c. Ascorbic acid (vitamin C) is a reducing agent and just by holding the tablet on the stain the chemical reaction will lift it in less than a minute, If it does not the stain is not metal (or at least a metal that is easily treated with ascorbic, citric, or oxalic acid).