The aquarium is nexus of power that would make gods tremble in awe.
More specifically:
Some millennia back, a wizard built a perfect magic insulator, in the form of a clear box with a lid and installed enchantments to slowly accumulate external magic and manifest it internally in a pure liquid form. This was to be a grand source of power at some point in the future, but, as is so often the case, the wizard's plans to avoid death didn't succeed and the experiment was forgotten. many, many, manymanymanymanymany, many years later, it was rediscovered, as a clean box mostly filled with liquid that seemed to be water. Someone had the bright idea of filling it the rest of the way with water and putting some fish into it. Things were a bit chaotic at the time and with the fish living in the water and drawing on the magic to meet their needs, no maintenance was required, and nobody thought to do any when the need was not apparent. Over the ages those fish shed their physical bodies and the water was corrupted into nonexistence and replaced with liquid magic. Deciding that the whole deity thing seemed a bit dull, the fish have been lazing around watching the chaos on or around the ship while maintaining the illusion of a simple fish-tank by teleporting any overflowing magic and causing anyone who might discover them to forget what they were doing. While they possess the aptitude and intellect to control the magic, and there is enough concentrated magic there to erase a continent, they have come to an understanding that interfering in things tends to make them boring. About the only evidence of their true nature is a tendency of people who think poorly of the goldfish's memory to succumb to memory problems of their own...