Uhh... I'm pretty sure the seahorse takes up no space. There has been a very good discussion about what people think seahorse does, try searching for them.
![wink ;\)](/ubbthreads/images/graemlins/default/wink.gif)
And buying ornaments increase the beauty of your tanks, as well as the number of customers.
EDIT: LadyCFII corrected me about the seahorse taking up space, read her post below.
![smile \:\)](/ubbthreads/images/graemlins/default/smile.gif)