Well, first off, you should try to find some references of some old looking fish to work from.(Probably not the easiest find.) This way you won't be totally lost if something goes wrong. It looks pretty cartoony now, but you could give it a little grey beard to show that its old. I found a pic that might help though. Notice that the scales are very pronounced and the skin is pretty wrinkly. Also, the fins are a lot bonier than usual. This first fish is 50 million years old, so dont go too far with this one.