To run the Goldfish Aquarium from the task scheduler, you need to use the 'start' command. Here's what I have in my "RUN" line in the task scheduler for it to run correctly on my Windows 98SE system:
C:\WINDOWS\COMMAND\START.EXE C:\WINDOWS\SYSTEM\Goldfish.scr
You might have to change the directories according to what version of Windows you are using. Hope this works for you!