if you want to create a natural enviroment that your frog should thrive in, you should use a soil substrate instead of gravel. in the wild, pacman frogs are found burrowed into soil or hiding in aquatic plants in shallow water or very near to water. they prefer moist soil in which they can burrow into and hide until their prey moves within "stricking distance". they are ambush predators and prefer to have their food come to them as opposed to having to activly hunt it, unless necessary, though many captive pacmans will activily chase food in their cage since they don't have very far to go. your best choices for a substrate are bed a beast, forest bed, jungle mix, etc. or sterile, untreated soils form a garden center.this would alos be a good time to include some live plants like pothos, arum, baby tear, spyder plant, etc. it would be good to leave the gravel in the bottom of the tank and put a layer of window scren or landscaping fabric over the gravel and then add 2-4 inches of soil on top of the scree or fabric and the gravel. the gravel wil act as a drian for the soil keeping excess water and waste out of the soil and away from your frog. this will alos help to lenghten the life of your soil, which may then only need to be claened every 3-6 months as apposed to every 4-6 weeks without drainage. the pic below shows a basic setup in a 15 gal breeder stlye tank with one 4" pacman.
as for feeding your frog just fish, i would strongly recommend reconsidering this choice. a balanced diet consists of variety, all animals should be fed a varied diet to ensure that the animal receives the proper vitamins, minerals, and nutrients that it would not get from one single food source. another issue with fish are that many times feeder fish are not treated with the same respect that show fish are and therefore are more likely to carry parasites, fungus or bacteria that could be harmful if not fatal to your frog. if you choose to feed your frog primarily or only fish, then tropical or cold freshwater fish are your best choice, but that can get expensive fast.
to ensure the frog receives proper nutrition, it is best to feed it a variety of insects, worms, fish, amphibains and mammals; including crickets, mealworms, waxworms, phoenix worms, grasshoppers, flour bettles, red worms, european and north american earthworms, rosie reds, guppies, swords, neons, gouramies, chichlids, bullforg, leapord frog, pickerel frog, green frog, and north americian treefrog tadpoles and froglets (i know it seems creul, but its for the good of your frog) and as they become sub-adults and adults, mice and young chicks can be fed if they are of an apporiate size in relation to you frog.
