Water filter issue

I have an under sink reverse osmosis water filter like this. My cold water line has a tee that sends one side to the cold faucet and the other side to the inlet of the filter. It goes through 5 filters: sediment filter, two carbon filters, the reverse osmosis membrane, and another one, then to a pressure tank. The pressure tank sends the water to the filter tap. I’ve had this for several years. I changed all the filters except the RO about 10 months ago.

The last few days I’ve noticed little black flecks in the unfiltered water coming out of the cold water faucet (but not in the water from the filter tap). I’m pretty sure this is carbon from the carbon filter(s), but I don’t understand how it’s happening. First, the filter should shed this much carbon; second, if it did shed carbon, it seems to me it would go downstream and maybe clog up the RO membrane, not swim upstream and come out of the cold faucet.

However, the tee has a valve to shut off water to the filter, and when I shut that valve, I no longer see the flecks. So that seems to pretty definitely confirm that they’re coming from the filter. Also, I don’t see the flecks at any other faucet in the house.

Any thoughts on why this is happening, and how to stop it?

Bad carbon filters.

The pressure tank on the RO end plus fluctuations in water pressure in your home, from home water use and from the street, and if you are on a well from your main pressure tank causes backflow from your RO pressure tank at times. There may be a check valve to stop this that is sticking/not closing. If that is the case just replacing the carbon filter may not work, as the new filter will just be back-flowed also.