Hey guys.
I guess this pops up a lot but here it goes.
I rendered a flip sim with a black BG and tried to comp it in Nuke using a blueish-whiteish BG. The renders have a halo. Is this a render issue or a comp issue?

This is usually a volume compositing issue. Unless you render deeps you will get a mismatch in sample accumulation when doing a simple over. The alternative to rendering deep is to hold out your other element by the volume and use a 'plus' type merge, however this limits the flexibility of the comp considerably. If that doesn't solve it then there might be some other issue.

try disjoint-over mode in nuke