In a 5g unfiltered ??? tank with two fish-it needs more water changes to maintain water quality...what about water temp?
I would increase the water changes-if unfiltered-1-100% and 1-50% weekly and if filtered twice weekly 50%
At this point I would Qt him and start a daily 100% water change with aquarium salt 1tsp/gal for 10 days
You have more that goes on in the water than just ammonia-a lot of it we can't/don't test for and water changes are the only way to keep it at safe levels in a closed system
You also need to rule out velvet(google pic)-and make sure that is not the cause of the clamped fins-