Pour a gallon of bleach down the well itself and see if that helps. Or do the same to the holding tank. Let it sit for a day and then flush it out by turning on every faucet in the house and outside if off that’s off the same system. That should clean out the pipes to the house. Treat the holding tank a second time if you’re still worried. I’ve been told to do this once a year but never do.
The old well is no longer in use, shallow water well we have now is ok just not great for drinking, but clear water and a good bit of iron. I put some bleach in it a couple of times a year to keep bacteria down, we generally use bottled water for drinking, anything else is usually boiled.
The artesian well I mentioned comes out of a horizontal PVC pipe beside a local road, no way to pour anything in and it would be immediately flushed out anyway, it’s been running 24/7 for many many years. Going to have to get something like a round stovepipe brush and try to clean it out, just haven’t been able to do it, no money these days. I’d love to get it usable, best water around here, I used it for about 10 years.
Public safety guy told a neighbor it was tested 20 years ago or more and found to be some of the best water in the area. Yellow looking stuff has to be growing inside the PVC pipe, it was never there for years until recently. They just can’t approve it for public use, bureaucrats can’t control it, add chlorine and flouride, “purify” it, pump it all over and mainly, charge for it.