Personally, I don't like to mess with the computer clock... Other applications that I'm running can get confused.

Also, I thought that the designers did a very good thing when they had the game take its time. This isn't a game to rush though. Breed the fish, wait to see them grow, see the results, breed again.

OTOH, I will use the 2x option to speed things up a bit when I'm actively checking, or the 1/2 or stop time option when I'm away.