I have yet to see a confirmed 15"+ specimen in anyone's tank. Most of the largest males we see on this site are around 12-14". They'll be fine for a very long time in a 75 gallon as long as the water quality is maintained diligently. Same thing with Jags. 18.5" wide is plenty big for either of the species and entirely doable. Now, is it ideal? No, but it'll do and it's entirely possible to keep and raise a healthy specimen in that size of a tank for life.
