I do try to tag my photos, but it takes a long time so I'm well behind!
However, when I started doing it I only noticed the function because aquarium was already so big! I actually try to avoid the 'aquarium' tag now, to try to restore some balance. The second biggest, 'waterfowl', is largely my own creation though - it just happened that Slimbridge, Blackbrook and Arundel are among the photos I've tagged so it got quite a boost.
I know Tim May tries to tag his photos - other than that I'm not sure how widely used this function is.
If Sim's reading, I don't know how easy it would be to implement, but it may be useful to have a browsable list of all tags that have been used - otherwise beyond the top 75 you're reliant on searching for exactly the wording used when the thread was tagged. Just a thought.