They're like those pens you had when you were a kid, that were full of water and glitter or something similar. The glitter moved around while you wrote.
In the tip is glitter and a watery liquid that the glitters or other small objects are suspended from. Looking into this trend I found you can buy kits with the nails already hollow, or you can build them yourself.
At first I was like "why the hell would anyone want that", then after awhile I kind of wanted them myself.
What do you think? Would you try aquarium nails? Do you think they're fun? Trashy? Cool? Let me know!
Personally all I can think of is the scene in the Simpsons where Homer finds the naked lady pens and race them all (damn you YouTube, why don't you have a clip of that?!)