Water does not become radioactive. Only the PARTICLES in it. Quoting: Anonymous Coward 50117919
Aquariums filter out the water.
But it's possible that the space dust is super microscopic, and gets through the filters.
Aquarium filters do not filter out radioactive isotopes. Come on now, they're built to catch algae and fish poop, not strontium-90 and friends!
The radiation is in the air, the water, it's everywhere. This sea star wasting disease has been seen before in sea stars - it just hasn't affected them on such a widespread level.
RAIDS is the best theory we have, anyway - it gave the wasting disease the foothold it needed in the sea star population.
Remember, it doesn't take much to upset equilibrium in homeostasis. Chaos theory and all that, you know?
(On a side-note, I'm a card-carrying ISON believer, so don't be offended - I think we may see PLENTY of comet dust in the near future).