Belief can be reinforced by evidence, but that is dependent on an individual's acceptance of that 'evidence'. If a strongly religious individual accepts their Holy scripture as incontrovertable truth, perhaps the direct Word of God, then that scripture serves as evidence to validate their belief. However, for many other people, as you have demonstrated on this thread, the evidence is viewed as weak / spurious / nebulous. In such circumstances the 'knowledge' is conditional.
In the case of the goldfish, once the existence of the fish has been demonstrably proven, the knowledge is incontravertable, ( except for pedantic whataboutery ).