skinny frog, not moving

I'm sorry if a similar situation has been answered before, but all the "skinny frog" threads I have read are about new frogs. I have had 3 auratus for a few years and their environment has not changed.

Someone else has been watching my frogs for a couple weeks, and when I got them back all my fly cultures were empty... hmm.. how were they eating? Okay I got my hands on some new cultures ASAP. One auratus was very skinny so I was just trying to feed them as much as possible. I thought about taking out the skinny one, but was worried about the stress of moving, so I left him in and just fed lots of ff's and springtails. This morning skinny frog was not moving. I was worried he was dead, but poked and he moved a little bit. I took out the 2 fat healthy frogs and put them in a 2.5 gal for now, so that skinny frog can be alone to try and eat. This afternoon he still looks bad. Same deal - not moving but moves if I poke. I did see him eat a fly this afternoon.

Not looking for cause (which I believe is not enough food) but rather what to do now? I feel like a very bad frog-mom and I am really worried that he is getting worse not better:(