You can change the time on your computer, but that may affect other applications so I wouldn't recommend it unless you REALLY know what you are doing.

I have a few other suggestions:

Start multiple games. You can have five games going at once. That way you can try different strategies and have more options to keep you interested.

Aim to get the Magic Fish of Growth. Have multiple ones of those in your "nursery" tank will help speed the growth of the young fish.

There's always the growth hormone, but that can get "expensive."