Stringy poo's and sunken bellies is a classic sign of intestinal worms you need to give them a bath in a bucket of aquarium water with the correct amount of wormer.
For example if you have a 20l bucket get 1 X drontal plus tab from your petstore or vet for a 10kg dog, Cush this and add it do the water. Add all the fish from the tank to the bucket and leave for 45-60 minutes, swirling the water once in between.
Catch all the fish place them back in the tank (NO NEED TO ACCLIMATISE) leave for an hour then top the tank up again
HTH
Ben