Hey there! My mother has 3 African Dwarf frogs (I got her into them lol) and the one is very skinny and lethargic and looks so under fed while the other 2 are pleasantly plumo after eating and do have great appetites and are active.
I help my mother isolate the little guy (his name is Ringo) and he is currently in a small aquarium and she has been giving him his own food but he just floats lazily at the top. He was very active but recently became more lazy (weak).
I am going to suggest trying other food as maybe he just doesnt like brine shrimp (if that is possible). aside from his size and weakness everything else looks normal.
The tank she had him in is a fluval edge like I have and it has a great filtering system and I bought her all the proper water conditioner and cycle (beneficial bacteria) to put in it.
I have seen frogs float on top of the water for a long time but that was a frog I had a couple years ago who apparently died from dropsy. Any help/input would be appreciated, thanks