Ok, so the consensus is the tank has to be full... There is 26 individual types (not including rare colours) presently, so the reason I've not seen them, apart from my Aquarium being permanently wafered now to protect the set I have, is that I never ran at 30 as I was spawning more to get the set previously... but wasn't the coming Publish (when they've finished ruining crafting with more complexity) looking at expanding the Aquarium? Worth bearing in mind if you're chasing shells, it may not be the same or even possible after they've changed the Aquarium to get them. Have one and cram it full of 30 not-rare fish then!