Don't use white. I don't know the reason why white quickly develops cyano but it always has for me, and balck never has. BRS used to carry black and shipping is free.
I understand that but Nems in general often don't show distress for many months once a "slide" starts" and they are unhappy. Certainly not saying that's the case here, but they can be delicate and keeping them in optimal conditions prior to a move to a new tank would make sense if it's easy...