So, I have a whole 3 pages of background and musings, but I'll really try to condense it.

Pond fed system, pump uphill 160' elevation to storage tanks via 900' of 2.5" PVC Rainfall is only source
(inverter powered 240VAC 1/2hp pump, enabled 11am - 4pm via timer & float switch in tank)

water test 3 years ago showed 1ppm Iron, .3ppm Manganese. No stains in plumbing fixtures. (new testing being run now, 10 days for results)

Separate domestic system fed off of the main storage, first is upflow gravel rough filter, then a downflow slow sand filter.
Awesome water quality 8 months of the year.
Filtered water stored in 1,500gal tank with ozone bubbler in it, then gravity feed to house at 60psi.

However, later in the summer, as pond level lowers, I believe water in hillside next to pond, drains into pond, and
then it all goes to red slime. Filters plug, water tastes nasty, bacteria levels skyrocket, I have to chlorine shock the tank and water lines,
scrap $200 filters that wont wash clean (even weekly washing won't work). I think iron & bacteria feeding on it,
overwhelm everything and the slow sand filter plugs and won't flow. Have to use a garden cultivator to break up top 1.5" of sand, which has become like cement, this destroys the schmutzdecke and takes a month to re-establish. It will flow another month and then plug again.
I suspect this is all iron bacteria. I can run water into a bucket, and it looks clear, but next day red speckles on bottom.

The slow sand filter processes water at about 5-10 gph, hence the storage tank after it.

I am thinking a way to trap/stop the iron / bacteria, is to use a smallish tank in the 100 gal range, and aerate it with a aquarium air stone, only using 20 watts or so. Following this, use a cheapie 10" cartridge 20 micron filter to grab the iron bits, and then the de-ironed water goes into the slow sand filter for pathogen processing. Water would be aerated for quite some time in the 100 gal tank, since it's only moving at 10gph.

Would a 20 micron filter stop enough of the precipitated iron at that point to solve my problem ?
Would plain air be enough to precipitate iron ?

I don't have the power or clean water to backwash greensand tanks, I can only clean water up to 15gph with the slow sand, using 200 gal for backwash puts me in the red !

I need the slow sand to be the absolute filter agent for Raccoon Roundworm Eggs,
which are prevalent in California wild areas. (this new 20 micron filter element would likely get a majority of the eggs)

Thanks in advance, Mike