1 - insufficient light
2 - insufficient co2

or

3 - water circulation too powderful the bubbles keep bein blown off
better quarantine.. if im not mistaken its contagious..

1 - either u buy rm15 medicine and try to treat them
2 - or use that rm15 buy new cories 

@Vampyr3, You got yourself a pair of Sterbai cories. I got a pair, and from the pictures, IMO is just nothing. I don't think its itch, but just to be on the safe side the quarantine would be good. But one advice, don't put salt in their water, it'll kill them, you might read from somewhere that salt would be a good treatment for itch but please, it don't include cories...

@cwl_ykh, as the saying goes, big fish eat small fish. Fish will eat anything that can fit in their mouth, and shrimps will be especially vulnerable when they mold. I loss all 30 of my RCS to my one dozen harlequins in one month's time. But it differs between tanks, some say no problem, some say all eaten. Its up to you to see if you want to try...

hehehe
some fishies wont kacau shrimps.. but every fish will makan shrimplets

make sure not cooking salt.

i used epsom salt.