I wonder whether a "washout", commonly thought of as something done over a day or two to the boiler interior, extended to the tank as well. Any boilermakers here?
The fish Smith wrote about that the driver kept in the G&SWR tender tank was an eel as well.
If St Philips Marsh, Bristol, needed to do the same it may well have been the Taunton water supply. Both the troughs at Creech and all the columns around the station and shed were supplied from pumping houses drawing from the adjacent Bridgwater & Taunton canal, which the railway owned, principally it seems for water purposes. No longer used for navigation, this had a thick surface of weeds along its length, and much as it may have been filtered at the pump houses there must have been some got into the locos.