Well. It turns out the airlines are very, very, very, busy. So they don't really have time to clean the water tanks. Well, they do 4 times a year ~ Keith Connors
Try not to gag. Check out the story below and from FOX here.
You might want to avoid drinking tea on an airplane. Researchers from Hunter College's NYC Food Policy Center polled 11 airlines, including JetBlue and Delta, about about the maintenance procedures for the water tanks of their planes. Charles Platkin, a professor of nutrition at Hunter College says, "Planes come in, [and the tanks are] not being emptied and cleaned, because there is no time for that. The water tank is being filled on top [after] each usage. Whatever would be on the bottom stays there and sits there." The standards surrounding drinking water on flights by US-based airlines stems from the Environmental Protection Agency's (EPA) drinking water rule, which relies on self-reporting by the industry and requires that water tanks be cleaned only four times each year. Platkin adds, "[The rule] was instituted because there were issues with coliform, which is a broad class of bacteria. I don't want to freak anybody out, but it's feces. [In 2004,] the EPA did a test of airlines and found 15 percent of the aircrafts tested positive for coliform." He also says that because of pressure on crews to have quick turnaround times on the tarmac, he worries that maintenance of the water tanks is not a top priority. If you want to steer clear of E. coli and other pathogens you will want to avoid getting beverages that are brewed in-flight, and you may want to consider bringing something to disinfect your hands with after using the plane's lavatory rather than washing with the tap water.