What could cause my water to taste metallic after a water filter cartridge was changed?

About 20 years ago, we remodeled our home and had two water filters installed (downstairs and upstairs). They are stainless steel enclosures with carbon block filter cartridges inside. The brand is MultiPure; after 20 years the model names are different and the design might be a little different, but this appears to be the same product: Aquaversa

I live near Seattle and our water quality is excellent. The water is river water from melted snow; it is so soft that they actually add some minerals at the treatment plant (to reduce the mild acidity of very soft water). We like the filters because they strip out the chlorine and the water tastes better.

I just replaced the filter cartridges in both filters. Now I am finding a strong taste of metal in the water.

When I went to replace the filter cartridges, the quick-fittings connecting a water hose to the inlet developed leaks. (I mean that on both filters, the same fitting developed the same sort of leak at the same time! I believe this was due to the inevitable yanking on the hose when I took the filter assembly down from its mounting, to open it up and replace the filter cartridge.) I wanted to make sure this sort of leak wouldn’t happen again, so I used brass compression fittings. I am pretty sure that the brass fitting is not part of the problem, but I am mentioning it as possibly relevant data… let me know if I am wrong and this is part of the problem.

I opened up one of the filters and found what appeared to be rust or corrosion. I polished away the visible corrosion using Astonish, a non-toxic blend of silica flour and detergent. The metal taste is very much reduced but not gone.

Before I polished away the corrosion I took a photo with my cell phone.

You can see that this corrosion is sort of in a line. The picture shows the upper half of the filter enclosure; I believe the lip of the bottom half lines up exactly with that corrosion when the two halves are assembled together.

The corrosion was not all the way around; it was mostly on one side so the photo shows all of it. I’m not sure but this might be the side opposite the bracket where the filter hangs on the cabinet wall under the sink.

After I polished away the corrosion, I rinsed the filter a lot, wiped it out with paper towels (not seeing any grey residue on the towel) and then let the filter run to waste for 20 minutes. When I was done I tasted the water and the metallic taste seemed very much reduced.

As far as I know the enclosure is solid stainless steel. Stainless steel isn’t impervious to corrosion, just more resistant than normal steel… is that correct?

So, what is the most likely explanation for the metallic taste in the water? Should I try removing the brass compression fitting and using a plastic fitting again? Should I take apart the filter again and polish it some more? Should I try to get the manufacturer to send me a new enclosure? Should I get some sort of test done on the water to identify what metal I am tasting?

One problem I have: I test the water by tasting it, but once I have tasted the metallic taste, it stays with me for about a day. So if I try things to reduce the metallic taste, I can only try about one thing per day, unless whatever I do succeeds.

I guess one thing I should do is to open up the other filter and check it for visible corrosion. So far I only have checked one.

EDIT: I have been thinking about this. In addition to the corrosion I showed above, there was also some corrosion on the bottom of the filter. There were some “nodules” of corroded metal, as if some chunks of metal had fallen to the bottom and settled there and then corroded. (In normal use, water enters from the top, then flows through the filter cartridge and exits from the top. The bottom won’t see much water flow, so the nodules could have been undisturbed despite using the filter.)

When I rinsed out the filter enclosure, the nodules broke apart and most of the corrosion was rinsed away. I put the filter in, tasted a strong metal taste, then took it apart and cleaned it as described above.

I’m thinking that maybe flakes of corroded metal from the broken nodules may have gone into my brand-new filter cartridge. The carbon block does nothing to remove metal from the water; it strips chlorine out but doesn’t touch minerals and such. So, tiny flakes of corroded metal could be slowly dissolving into my water.

This theory would explain why the taste started after changing cartridges (because that’s when the nodules broke apart), why the taste was reduced after I polished away the corrosion (because I got rid of the main source), and why the taste is not completely gone (tiny flakes stuck in the filter cartridge).

Therefore, I’m wondering if the solution is to take the enclosure apart again, make sure the entire inside is clean, and put in a brand-new cartridge.

Since the cartridges are expensive: if my theory is right, I wonder if I can flush out the metal flakes somehow, to salvage my otherwise-new filter cartridge?

To test my theory, I could put in a new filter cartridge… but could I then remove it, dry it out, and put it away for the future? It seems expensive but safest to just put in a brand new cartridge and leave that there. I’ll be sad if that doesn’t fix the metal taste, though.