You said you are feeding Pleco tablets, Pleco tablets are usually for fish with a more omnivore diet than BN's and have more protein than is desirable, especially for young fry. By the sound of it the ones that appear bloated have got the lions share of them.
Stop feeding the tablets but keep up with the peas and courgette, hopefully the effected ones will pull through, but to much protein at an early age is dangerous.
I tend to stick with softer foods like cucumber and courgette sometimes lettuce for the first couple of weeeks untill they are more physically developed.
Good luck with them, hopefully you have caught it in time.