Lets first address the fish. A true Texas cichlid, H. Cyannoguttatus(sp?) is the only Texas cichlid. What folks refer to as Green Texas are Carpintis. Nothing Texas about them. What folks refer to as Red Texas are man made hybrids of who knows what, but at least one parent should be a true Tex. You can name a fish anything you like, but a lot of what is referred to as Red Tex are plain Flowerhorns. To the question... 90g will hold a pair of Carpintis, but a 48x18 footprint is still tight. H.Cyanno are much bigger, and 90 will be too small for a pair, imo. Red Tex hybrids can be bigger still, especially if having some Red Devil/Midas in the mix. For certain Red Tex is far more violent, ergo more difficult to house in pairs or groups. 90g wont cut it imo. I have had all fish in question, I offer an experienced based opinion. I am sure some will say 90 g is huge, mix them all in, no probs. They would be wrong, imo,lol