Someone posted about this problem over at the Astraware website. Kieren, one of the technical support wizards, had this to say:
This is unfortunatley a known issue with the current version of Fish Tycoon on VGA devices, and one which LDW do plan to fix in the next update.
The cause of the slowdown is because Windows Mobile 2003 Second Edition, as well as adding support into the operating system for VGA screens, also includes some routines to help display applications designed for the "standard" QVGA (Quarter VGA, or 240x320) resolution on the larger screens.
Unfortunately, these default scaling routines are not designed for gaming, and so tend to result in the game running much more slowly than it was designed to, as you've seen with Fish Tycoon.
Most often, developers need to implement their own scaling systems to get the speed they need, but it can sometimes be a little tricky and time consuming to do.
It may be worth dropping support@ldw.com a mail to see if they can give you an idea of when this issue is likely to get fixed, but I do know they're working on another big project at the moment, so unfortunately it could yet be a little while!
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Hope this helps.