Re: Dark Brown water - NOT IRON!!!!
Ben is, of course, right. And, yes, I did run an Intex Donut for a few years before installing a FantaSea 40'x16' pool. Intex filters are fine for clean clear water, but not so good when you have problems...been there, done that.
I seem to remember somebody inventing a contraption using a submersible sump pump and a capped 5 gal bucket filled with cotton()? that acted as an adjunct filter. It sucked the metal out and it stuck in the cotton...does anyone else remember this gadget? Real Rube Goldberg but it worked. I'm not sure what else might work for you. If, in fact, it's metal (and I'd bet it is) the more you can filter out the better.
When I had my Intex I jury-rigged a vacuum that went through the filter cartridge. I had to be creative to find a way to use the drain for the vacuum but it did actually work.
16'x40' rectangle 19K gal AG vinyl pool; ; Hayward T210 sand filter and 2 Speed Superpump 1hp pump; Solar heated. Test kit: PS 233; PF:6.3