To increase your number of shells exponentially, you'll have to go into your system files.

First, make a whole ton of users in the game. Ex. name them USER 1....USER9. Play as one of the users and get some shells (complete the first tank). Quit the game. Then look for the directory where you installed Insaniquarium, and
go into the userdata folder. Copy user1.dat and make as many copies of this file as you have users in the same folder. Ex. if you have 9 users, copy user1.dat 9 times. Then rename them to user2.dat, user3.dat... etc.

Run Insaniquarium. Type 'give' in the menu screen and you'll see a box with all the users in it. From there, you can transfer all the shells to one user (preferably the first user). Close the box, and then in Meryl's voice bubble, click 'If this is not you...' and change users. Repeat shell transfers.

Quit the game and, again, copy user1.dat and repeat the renaming process.