Hi Tanny,
To make a aquarium nail you need to stick 2 clear tips together with L+P or clear gel along the side walls and the free edge, leaving the well of the tips open to allow you to fill with water and glitter.
Once you have added the water, glitter and whatever else you want to put in them, you seal up the well with more L+P or gel.
Keep in mind though, these nails are quite thick and can be bulky, and are more of a novelty and most clients would prefer a proper enhancement.
hth x x