sorry to hear this. Pleuros are indeed very susceptible to sudden death syndrome. I've raised about 12 from juv stage and lost 4 to sudden deaths. I dont know if this will help but when i kept them i had them at 28C, ph of 6.8, low light, and somewhat high current.
Just a theory but i only lost my pleuros to sudden death when i had them in low flow tanks. I found that when there was higher current, they were more active ate more readily and seemed more healthy.
the current if very strong, the tank temp is 26degrees ph is 7 give or tak a little and there is very low lighting, the guy in the lfs is giving me a call today so hopefully he will know whats wrong, he said he will be giving me £50 store credit so im wondering what to buy with it