I would just pause the game!

The way you do that depends on which version you're playing, but the game speed is accessible by going to the Menu screen and then selecting Options or Preferences. In the Windows and Mac versions, Stop Time = Pause.
If you're playing more than one player slot, there are some precautions to observe once you return to the game. Due to the way that the game updates the state of your tanks, you should always click Play immediately after changing players. If you don't, all of your fishies could die.

After you've clicked Play, it's safe to return to the menu and change the game speed.