I'm pretty sure I've trained a FW angelfish. I feed blood worms from a small plastic cup. When I started doing this, she would occasionally peck at the bottom of the cup before the food could start to flow. Over time, she learned that if she positioned herself downstream, the food would come to her (no need to peck the cup).
Not to be outsmarted, I started lowering the cup into the tank, but wouldn't let the worms flow unless she pecked the cup first. It took a little time, but she figured it out. Cup goes in. Peck the cup. Food comes out. It's become kind of a ritual. I guess I'm easily amused.