I would say 125g, however I've never owned this fish. Tangled up in Cichlids and Cichlids of America I think have them right now for sale. I've went through TUIC on a few occasions, but never went through COA.
The only place I would recommend to buy black nasty (haitiensis) is cichlid connection/don conkel tropicals, I've bought from almost everyone on here and everything died of bloat, except the ones I bought from conkel. The haits I bought from there are the most healthiest I've ever seen and so are the fry, not one single bloat issue with that bloodline period. I will never buy haits anywhere else unless they were previously bought from conkel.
Ira from the wish list also buys haits from conkel and he also has growouts from one of my original pairs I purchased from conkel
i had two in a 125, not a pair, both males, the larger was about 7" the smaller about 6" and i couldnt see a 125 being big enough, especially considering a male could get to 16", id say a 6x2x2 or a 180 would be ideal