I understand that the ink is supposed to be non staining, made of mostly tap
water, etc,

I wish. The water is not cheap. It's distilled..and then
some...etc...etc...it is as pure as water can be... Thus a big reason my cost
for ink is the highest, and profit most likely the lowest (competitors have not
showed me their books! ;-)

For rinsing out my 61's - I actually put the filler end in a surgical tube (the
stretch latex kind) and put the other end over the faucet - then run the water
through the pen at low pressure for a few minutes - until the water runs clear.
You can also do this holding the filler end with your hand sealing enough to
push the flow through - but you'll waste more water that way...and might spray
it clear out of the sink and make a mess.