To catch a Robot Fish you need a bait of piranha.
To catch Piranha, you need a bait of minnow
To catch minnow, you need bait of apple.
Just buy an apple from the grocery store, or grow your own.
Makes sure to inspect the water before fishing so you know
you're fishing in the right place!
Robot fish can only be found at the pond behind the Science Facility!
Wow, you guys actually use bait to catch fish? Most of the time, unless I'm after deathfish (which requires angelfish as bait) I just use the lure. Once your fishing level is high enough to catch the robot fish, you can probably catch it with or without bait. The big thing is to have a high enough fishing level to catch it in the first place. Skill level for Robot Fish is fishing level 9. That will go a long way in getting the fish you need.
Your Sim can learn all fish baits by buying the books at the bookstore that teach them the baits, but even there, you have to be level 9 fishing to even read the book that includes Robot Fish.
I've spent over $2000 on store stuff, the LEAST EA can do is make sure it is functional!