Swimming robots is an interesting area to give a try. If you have already built robots that walk, ride on a dry surface, then to fill the gap, you should make one that swims. Boris Landoni have made a pretty simple construction robot that mimics the fish in the tank. It’s construction is fairly easy – An Arduino with three servo motors controlling tail, fin and torso.
There are also couple sensors that are used to detect obstacles. The infrared sensor does the job pretty good. Fish propels itself by creating harmonic motion with all three servos. It looks pretty realistic and with proper skin on body you may be fooled.