Yes -
"Settings"
then "General"
then "Auto-Lock"
and change the Auto-Lock setting to "Never".
Don't forget, though, that this setting affects all of the Apps and you'll be going through battery power a lot faster.
It would be great if the iPod Touch had separate settings for when on battery or plugged in. Maybe that will be on a future update to the OS?
What are the chances of having an updated version that provides an option in the app's settings to disable Auto-Lock while running the Aquarium App? I know other apps have this option in their settings (example iRedTouch). I typically run Aquarium when my iPhone is docked and I don't want to turn off the entire phone's auto sleep / Auto-Lock , but would like the option to disable it in the app itself.
I also wonder how the app will look on the iPhone 4's better res screen.