Hard to say really. Some people's "pinches" are bigger than others!
Any fish food should do the trick. I'd add a medium sized pinch every other day or so, and start testing for ammonia after a couple days. Once you see the ammonia show up... say at levels around 1.0ppm or so... then stop with the food and just let nature take it's course.
If worse comes to worse, and you end up with too much food, and ammonia levels over 4.0ppm or so, then just do a partial water change to bring the ammonia levels back down a bit. You really can't screw anything up.
