You know what,I actually believe a marine tank, specifically a reef tank, is one of the best things to use to entertain and to stimulate a very young child.
When my youngest was less than a year old, we used to put her in front of the marine tank when she was restless and crying and everytime, within a couple of seconds she would be quite and relaxed and watching the tank very carefully.
It was just amazing the effect the tank had on her. Perhaps we were just lucky, but it calmed her and kept her interested in it for a long, long time.
Manderinman - I agree with everyone - your daughter is too sweet for words! You know what - we do the same with my sister-in-law's kids when they come and visit (about a year & half and a few months) - put them in front of my tank, and they stay quiet for a long long time - and the next moment we see they are fast asleep!