What can tropical fish eat if you don't have fish food.
There really isn't a good substitute, but fish can go a few days without food, just skip feeding them and go get them fish food tomorrow.
Some alternative foods that you can feed your fish on include lettuce, peas, Brussels sprouts and shrimps. This should only be once in a while since this does not satisfy all the fish's requirements. See more details at http://www.aquaticcommunity.com/fishfood/alternative.php.
Like Deana Wynn said they can go a day or two without food so I would just skip the feeding and get the correct food soon.