Your water change routine sounds fine and you said that your tanks are heated so I cannot understand what the problem can be though I doubt t has anything to do with Nijinsky. Try leaving the cardboard between his tank and the bettas but honestly even if he was flaring a lot that would not kill the betta next door.....
You said that you mainly feed them brine shrimp and bloodworms do you mean frozen? Frozen and freeze dried brineshrimp have very limited nutritional value and should only be offered as an occasional treat. Fish are unable to digest the casings of bloodworms which means that they too are not a suitable diet for bettas and can only be offered as an occasional treat.
The only way to provide a stable nutritionally balanced diet for your betta is to feed them pellets. Some bettas dislike cheap pellets as they are poor quality so I would recommend changing the brand of pellets that you use. Even if the pellets you offered them are not cheap ones a different brand could still make all the difference. Also you need to let your fish get hungry when you offer them the pellets for example if they refuse the pellets don't go and feed them bloodworms/brineshrimp straight away otherwise they will just wait for the good stuff.
Sort of like offering a child their dinner and if they don't want it just give them lollies/chocolate instead
Goodluck with your bettas, hope they are all doing well soon