Frog poop, lol. . .
Yeah, frog poop seems to be less like fish poop and more like, well, frog poo! It can float, or not, but more or less seems to dissolve into the water itself, so from what *I* have seen (or not seen) it's more about water changes and less about getting the actual detritus out, like it is for fish.
SKINNY IS LOOKING GOOOOOD! If you have a male and a female, then she will always be bigger, so don't worry about that. As long as he's eating and plumping up a bit,I'm sure all is well. And I don't know about the kneecaps, I was concerned about my little male as he also had them when I brought him home. Maybe it's a male thing, or has something to do with age? Or maybe it was from being kept on gravel at the fish shop - I'm not sure. My little Freckle's knees are still a *bit* on the pale side, but have gotten WAY better since I brought him home. I'm betting your Skinny's knees will darken up too, as he grows up and has a few skin sheds :)