Well expansion tanks

As a rule bladder tanks don't have to be flushed out. Why do you ask? Is it not working? Is the pump cycling on and off? Please explain your complaint in detail. Regards. Tom

Thanks for your time and help ! The reason I was asking ......... I'm trying to help this disabled person get good drinking water. He is on a well and has a black substance leaching into his water. I looked at the supply line up to the expansion tank from the well .
It had a little brown rust in it but no black. The piping thruout the house has a slight black look inside the pipes. I have flushed and chlorinated and instahled a whole house filter and a water softner and still am getting a black mixed in the water. Is it possible the bladder on the expansion tank is leaching ? What other possible causes ?
I have concluded that the source of the black might be that bladder gone bad. I've seen it before when I was a kid ?
P.s.
If need be where would be the best and cheapest place to get another expansion tank ?
If I need to explain myself better let me know !
Thanks again for your time and help !
Tim robertson
Sheridan indiana

Any plumbing shop or supply house should be able to furnish you with a expansion tank and I think your analysis is correct. You may have to flush out the water lines and pull and clean each aerator. Good luck, Tom

Help! I'm not quite sure my analysis was correct. I went back and disconnected the line coming from the well and turned the pump on to get a sample of water straight from the well. It was black as oil and stunk like rotten eggs. So that told me my problem is coming from the well . The well pipe is a iron 6" pipe that I was told runs approximately 250 feet deep with the actual pump sitting somewhere around 120'-150'ft. I was also informed that this line when drilled had to go through some limestone to get to the aquifier.
Questions : 1. Is the black I get in the water a sulpher bacteria or an iron bacteria. An initial test for iron shows none or low levels.