Short version: Clear but green water after maintaining TC above 10 for 48+ hours, any chance this is caused by phosphates, or maybe copper?
Long version (photos attached):
I have inherited a 7500 gallon above ground pool from the previous owner of my house, with DE filter. Owner moved out mid-summer last year, so pool water (and equipment) was in bad shape this spring. I also inherited an assortment of Leslie's chemicals in the garage, making it obvious the previous owner wasn't doing BBB. This means I have been a bit cavalier with chemicals because they haven't cost me anything (except some Borax I had already lying around). Took the cover off two weeks ago, spent a whole day just scooping leaves, then...
Opened filter, cleaned out old DE that was left in it, hooked up hoses, turned on filter. Immediately added Leslie's Fresh n Clear because it was labeled shock, only now I realize it doesn't have Cl in it (Potassium Monopersulfate), which explains why no Cl registered in initial tests.
With the first test I realized pH was way below 7 (was testing with a few old strips at a time until I got close enough to care). Added pH Up a few times, pH was moving slowly, added Borax to keep raising pH after TA was in a good range. Finally got pH to 7.4, added 2 lbs of Chlor Brite (dichlor). Cl (OTO test) turned dark orange, and stayed there for at least 2 days. Vacuumed a few times, and opened, cleaned, and re-charged DE filter twice in that process, with trichlor tabs also in during that time to be sure Cl didn't drop too much during the days. Finally rid of most of the muck.
Water was then "clear" enough to see the bottom, but the color of iced tea. Started reading up on possibilities, I was pretty sure it wasn't algae due to water clarity and OTO test staying quite high. Vacuuming sand-like stuff every time it settled on the bottom, another filter cleaning (and filtering through an old T-shirt stuck in the skimmer a few times) took a lot of brown stain out of the water (not sure if that was iron or dead algae). Remaining stain makes the water look green: green tea, Mountain Dew, Ecto Cooler, Gatorade, whichever is most familiar to you.
I had some Natural Chemistry Metal Free, which I read somewhere is EDTA and would not hold up in high Cl. I also had Cl neutralizer (sodium sulfite is mentioned on the label). Added neutralizer a few times in two days until the OTO Cl reading was down to 1. Added the amount of Metal Free that I had, which was about half the the dose recommended for pool opening. Brown tinge was out by this point from filtering, but treatment didn't help the green. Also found Leslie's Super Stain and Scale Preventer which mentions copper and iron in the uses (a pink liquid, maybe phosphonic rather than EDTA, maybe similar to Jack's the Pink Stuff?). Added this also in a little less than the does recommended for opening, but didn't take away the green.
Had a water sample tested at the local pool supply. He said it showed no copper, but 3000 ppb phosphates, and claimed this is the problem. He suggested SeaKlear flocculant, but when I mentioned I had a whole shelf of Natural Chemistry "Pool Magic + Phosfree" he said try some heavy doses of that before spending $40 on the SeaKlear. He said it's not as strong but may take some of the phosphates out. Added 40 ounces or so and no change in color or obvious precipitate yet, but if I push a brush across the bottom I can faintly notice a faint dark cloud scattering here and there as the brush pushes by. I think I was seeing the same thing before adding the Phosfree though.
I'll admit I haven't sprung for the fancy test kit yet, but my partial current readings as best as I can tell are TC 1, FC ~0, pH 7.6, TA 120, CYA 40. None of the sand-like stuff settling on the pool floor any longer.
I currently have two trichlor tabs in a floater to raise Cl after the unsuccessful metal treatments.