Sounds like worms, TB spreads reasonably quickly and shows a more of a defined bend in the spine rather than hollow stomach.
Worms in fish are just as commin as in dogs and cats. You will notice that after you have fed the fish within an hour or so that the fish will look skinny again.
Crush up one drontal all wormer plus tablet for dogs upto 10kg. Add to a 20L bucket and stir into 20l of aquarium water.
You need to catch all your fish out and put them into the bucket, if you have many fish divide them up so that you will not put them through to much stress. Leave them in for 40 minutes, NO LONGER, remove any fish show signs of stress imediately.
When you net the fish out have a close look in the net and you will see heaps of little hairlike worms, EW.
Worm eggs can be excreeted by the fish and can live in the gravel until picked up by the next fish, dang! These pesky critters can also be carried by snails... so when you get some new plants there they are again, and new fish can also re-introduce them
Do a repeat dose after a few months if you suspect a repeat infestation.