iron stains in toilets but iron filter seems to be working well

New construction house, 3y old. Southwestern Ontario, in Canada. We are on a private well with fairly high iron (3mg/L) and hardness (612mg CaCO3 /L). We knew this when we built the house and installed higher-end iron filter and water softener. Water testing after the treatment system shows under 0.5mg/L and hardness of 76 mg/L.

In the last three months we have noticed that our toilets are starting to get brown staining consistent with high iron. We tested the water again and received results consistent with earlier testing. The rust-coloured deposits are easy to clean off the porcelain if done relatively soon after appearing but new staining returns within about 7-10 days. This is a new development; we did not have this problem previously.

On a hunch that perhaps the Clack valve on the iron filter was malfunctioning and it was perhaps not fully in backwash mode and thus occasionally backwashing the high iron from the media into the downstream water rather than the drain, I had it replaced last week. I haven’t had the water tested again, but using simple iron test strips report that I’m still seeing approximately 0.01mg/L which is really low, but my wife took a bath today and the water was quite brown.

(not sure if it is useful data, but these test strips only indicate the presence of Fe+2, so I have performed the test twice: once with water from the test site, and once again with about 1/4tsp of ascorbic acid mixed in to reduce any Fe+3 to Fe+2. The untreated tests show almost no iron at all (no pink colour) but the treated tests all show approximately 0.01mg/L, the second-lowest indicator available with the test strips)

The natural gas hot water tank is new. It’s rented, and was replaced last year because I asked the company that owns it to replace the magnesium rod to combat the typical “smelly hot water” problem; they couldn’t get the magnesium rod out so replaced the entire tank and installed an aluminum rod in its place. This helped the sulfur smelling hot water considerably but I still find I have to purge the tank every 2-3 months and put 1.5L of hydrogen peroxide into the tank to kill off the bacteria.

I have noticed that the tank seems to have a LOT of iron in it; every time I bleed the air out of the lines after treating the tank with peroxide I get nasty brown water for a minute or two from the hot water taps. I’m not sure how this iron would backwash into the cold water to the extent that it would get into the toilets, but this iron is coming from somewhere and I’m at my wits end trying to solve this.

Does anyone here have any suggestions on how I can identify the source of iron and even better, how I can solve this problem?