Yes, I might sell a few more copies if I allowed the user to create their own objects. But this would not improve the Aquarium, it would ultimately hurt it and damage my credibility. Anyone with the skill to make a creature matching the qualitly of the rest of the tank would certainly have the ability to create their own program.
Sorry, I do not want to re-invent the wheel. Any attempt I made at trying to create an entirely new Aquarium would be a disaster. I would give up after a few weeks of trying to recreate the programming you have done. The motion would be a joke.
My interest lies in contributing to the only game in town.