Red Eye Tetras With A Disease

what else have you got in the tank?

it does look alot the same thing that was wrong with your other fish, how big is your shark ?

the cause of the problem is most likely about 3 inches long with a red tail.
it may very well have got along with its tank mates when first introduced but is now becoming aggressive as it matures.
  good luck.   

i was naive enough to put a female black paradise fish in with some platys. after a few days it started to assert its territory and become a little aggressive to the platys.
as soon as i notice repeatedly chasing one of its tank mates from one end of the tank to the other i returned the black paradise fish to the lfs.
one of the platys sustained a small bite, this then developed a slight fungal infection which was followed by ich or white spot which spread to all the other platys.
as soon as i noticed this i started treatment, stopped feeding for two days, kept up regular water changes as i am fish in cycling and removed the carbon elements from the filter while upping the temperature a little.
all are improving rapidly.
i would let things settle down for a while and then research a suitable replacement for the shark.
good luck!