If its a well established tank you would be surprised how much there is to eat in the tank. They will forage in every crack there is to look in. If you can get some one to stop by in the middle of your vacation to feed once that would be best. Have the food measured out and hide all the rest. If it was me I would do a large water change and feed right before leaving. And do another water change when I got back and feed. Most likely they will be fine for 9 days. Only problem might be if you have larger hungry fish with smaller fish.