A 75 gal would only work for a while, maybe 1 year, but in the end they would need a 6ft tank(as others have said). My male easily hit 15", and the female about 11", and the pair maxed out a 150 gal tank in the photo below.
Nandopsis haitiensus are not your average cichlid, the waters they come from usually range in the high 80'sF to low 90s'F (read Hispaniola Oh Island in the Sun, The Dominican Republic by Alf Stalsberg)
http://www.cichlidae.com/article.php?id=55, and because they are the only cichlid endemic to their island, trying to keep them in a community with other cichlids, 9 times out of 10 ends up badly. I have raised hundreds, but lost my first 19 not adhering to their special needs. Once I learned (the hard way) they are durable fabulous species to keep.
They need very clean water, so lots of water changes need to be part of your routine. And because they are omnivores, I found a diet rich in algae based foods produced good results. They seem to be prone to bacterial infections when too much meat based food is given, especially if temps are not high enough to aid in digestion.