Which house's sigil is a fish? House Tully. And who is one of the (many) people who meets a bloody end on the orders of Tywin Lannister at the end of Season 3? Catelyn Stark (neé Tully).

As the scene was deleted, this one may hold a little less weight, though it was probably deleted only due to its length. You can watch the scene in full here – look out for Grand Maester Pycelle's moment of uncharacteristic honesty.