It's probably a bit hard to tell from this pic, but I'm just wondering if anyone can have a guess at what might have caused this stain... animal, mineral, vegetable? I have 3 of them now and my best guess is it might be either bat droppings or maybe a small leaf or seed pod that sat on the floor for a bit too long (although I don't recall seeing either). The spot in this pic is about 1cm in diameter and the other 2 are a little smaller. Colour is like rust I suppose. I've tried a vitamin C tablet on it but it didn't really do anything. I'll try a bit of dry acid in a sock next. What's the best method to get one down to the floor... stick it on the end of a pole or something?
One thing I'm wondering is whether it could be caused by minerals/metals? I'm switching to plain salt, and with all the recent rain I'd reckon I'm pretty much there. but as mentioned in one of my earlier posts, the pool was originally started up with a "mineral blend". My gut feeling is that mineral stains would be bigger and be situated more where the returns blow water onto the plaster (I've seen this at a friends house). Also, I really doubt the mineral blend had much other than plain old sodium chloride which is another reason I'm leaning towards animal or vegetable.
Thanks as always,