What type of frog is it? In the wild for most species of frogs, fish is not a common food item. It you really must give fish, you should raise your own. Guppies are a good choice, because they are live bearers who produce a lot of young frequently and are fairly nutritionally balanced (when it comes to fish and/or other vertebrate prey - ie mice, pinkies, goldfish etc.). Plus by raising your own, you help limit the chance of parasites.