Ask the student to walk through the logic in the fish looping functions, specifically starting at line 166. If they really read through the code and talk it through aloud I bet they find the bug.
function loopGreenFishL () {
if (greenFishL.x < 0 ) {
greenFishL.x = 400;
greenFishL.velocityX = greenFishL.velocityX +1;
}
}
function loopGreenFishR () {
if (greenFishR.x > 400 ) {
greenFishL.x = 0;
}
}