I would suggest you to divide your project in two part while developing:
1)Prepare your world simulation, start it in 2D, do not try 3D at first. Draw a rectangle and move it like a fish. This part will take you a while. You have to think to an ambientation, how to move the fish in a way that it looks natural. Then apply an image to the rectangle. You will have probably to flip the image when the fish change direction, from going left to going right. Start with one fish. When you get it right, have a look at "autonomous agent" (here a nice tutorial that you can apply to openFrameworks http://natureofcode.com/book/chapter-6-autonomous-agents/ ), and try to work with the movement of more fishes.
2)The second part is the scanning of the fish. You will have to use ofxCV for this. It is an addon that wraps openCV. This addon will allow you to scan the drawings of the fishes.
Once this 2 parts are done, you have to apply the texture of your scanned fish to the rectangle you've did in part 1. Do not think to finish this project in one weekend, getting ofxCV, getting the right movement of the fish and an ambientation that makes sense takes time. But with patience it is absolutely a feasible project, and a nice idea too