tilapia have kind of a "v" bone, i fillet just like yp, cut behind the head and follow the spine down, you'll feel when you can go all the way through behind the ribcage, then out to the tail, some people like to skin so only cut "to" the tail but not through, then use the tail to hold while skinning.. anyways, back to the fillet, cut along the ribs, let them guide your knife, you'll hit that v, i cut through it then remove after, and usually end up with 2 fillets from each side, a big one and a small one..
