Internal parasites or internal worms. General cure treats both, Metro only treats parasites, so use GC
I'm treating a TSB for the same thing right now
Mix General Cure with Seachem focus and frozen mysis, feed for 14 days. They eat mostly algae so I also made a paste of GC, Focus and hydrated algae pellets. I mostly dried out the paste with a fan and then froze it. I cut a slice of medicated frozen algea paste every day and drop it, and he gets medicated by both medicated mysis and medicated algae paste. He is better now with 5 days of meds to go
If he's not eating now, the outcome is not good, you must start right away