The Bubbulator allows you to have 20 fish in your Virtual Tank. You have the option to not display it under "Tank Options", but you have the extra 10 slots regardless.
Purchasing
The Bubbulator costs 20,000 shells, and is the first in a chain of one-off goodies. There's only one of each, and you have to buy them in order. You purchase the Bubbulator first (20,000 shell), then the Alien Attractor (45,000 shells), then the Vitamin/Food Upgrade (20,000 shells), and finally the Beetlemuncher, Cookie.