How about instead of interchangable coral, you let the user simply choose the color of the coral. Also, you could let the user change the backdrop color. I think it would really neat to be able to change the coral and backdrop to match your computer color scheme. These kinds of user options for how the aquarium looks are essential and will make it more fun. You could also let the user set the coral color as a texture aquired from a picture file, so companys could stick their logos on the coral and you can make the coral look however you want. Maybe you could also do these same things with the gravel so we can make the gravel any color or texture we want.If you used this idea, you could always post new bitmap textures to use for the coral and ones to use for the gravel. Of course, these things can only be done once the 3d coral is done. If I have any more ideas, ill be sure to post them!

P.S. - For anybody who hasnt seen the ms plus pack, I just saw it in a store and it has a nice picture of the aquarium on the front (among other things).