I recently closed the pool after our first season with it. After closing, I broke down my DE filter to hose off the grids and give it a thorough cleaning. While cleaning the grids, I noticed a black stain on part of two adjacent grids that wouldn't come off with simple rinsing.
Some relevant facts:
- The pool was a green swamp when we moved in, so I completely drained it and pressure-washed the plaster and then refilled it using the built-in fill pipe (essentially house water, which is from a mutual system--effectively straight well water).[/*:m:zq1w42cb]
- I took the water to Leslie's at the beginning of the season and they said I had some small amount of iron in the water (15 ppm?), but I skipped the metal treatment and had no problem with any staining when I did the initial shock after filling. The guy that tests the water for our mutual system said we have high manganese, but I haven't heard of that causing any problems in pools.[/*:m:zq1w42cb]
- The pool plaster may be the original, 30 year old surface. It has pop-offs in several spots, and there is rust showing through in a couple places. We got a quote for resurfacing, but the guy didn't seem to think it was any kind of emergency. (No sign of leaks--the pool holds water extremely well.)[/*:m:zq1w42cb]
- I installed the DE filter new before the season. It replaced a woefully undersized single cartridge filter. I have only ever used cellulose fiber in the filter (e.g., FiberClear, PuriFiber).[/*:m:zq1w42cb]
- I fought a green algae bloom starting at the middle of the season and I may not have completely killed it off--we continued to fight it (brushing, elevated but not shock chlorine levels, short filter cycles, etc.) for the rest of the season. I used trichlor pucks as long as I could get away with it (4 months maybe?) but have switched over and am now using 10% bleach. CYA is probably around 30-40 but I need to re-check it.[/*:m:zq1w42cb]
- There are (er... WERE ) live oak and maple(?) trees overhanging the pool and dropping junk in, especially in the spring. A lot may have gotten into the filter in the spring, before I put a skimmer sock on. I noticed some "tree junk" left on the grids when I cleaned it off. Some was bridging between some grids near the top, and that may be the source of the stain, but I'm not sure if the stain is where the bridging was. However, that same tree junk doesn't seem to be staining my skimmer sock, but maybe it just doesn't sit long enough, or the material's different and doesn't absorb the stain.[/*:m:zq1w42cb]
- The grids were not pristine when I was finished hosing them off--there was an overall brownish tint which I suspect may come from the oak tannins (or iron/rust?)[/*:m:zq1w42cb]
I don't have any pictures because it was late when I started and I finished reassembling everything in the dark. So what do the board experts think? Anyone have any idea what the stain might be, and should I be worried about it? I'd rather avoid acid washing on the filter's first season if I can help it.