Very small spec of crud can cause the valve to leak. I had it happen but now have two GE whole house filters. There could be pieces of plastic from the original holes drilled in the tank or while plumbing it to sand or specs of stuff that has dried on the inside of tank and flaked off.
I noticed small slugs of black crud when draining my tank one time. It was probably from well water in my travels and letting it sit just a little too long one time while visiting someone.
I removed the bottom panel and could see a dark witness marks around the inside of the tank. I then removed the tank and used a bunch of nuts and bolts inside and rolled the tank back and forth and all around to scrub the inside. It was a real pita but it worked. I now consider any tank other than new as needing a close inspection before I'd use it.
In fact after seeing how easily that happened to me I think there are probably a lot of tanks out there that people would be surprised to see inside of. It can't be just me.
I always add bleach as I fill my tank. I sometimes smell it at the faucet which is fine but we only drink water that has gone through our Berkey traveler table top filter.