You need 3 actuators, 2 Sharp GP2Y0D805PCB infrared sensors with digital output, 1 arduino Board, 4 batteries and common insulating material.
Can be done in sveral hours if you have everything prepared. Is not a big deal. The big deal is the code for arduino to help Robofish moving harmonic and detecting obstacles ... but this is also free on the open-electronics website. So you have almost all for your Robofish.
Only one question: Do you have time for build ?