I wouldnt put a Hippo tang in the 180 I have right now, but yet the firehouse down the block has one in a 55. I'm sure people put them in 75/90/100/125 gals and honestly I cant get mad at the ones who keep them on the latter part of that list.. but even a 75 against a 55 is a major improvement. Honestly I feel if you were to put it in your 75 you could be potentially saving one from somebodies 20gal.
Anyway, bigger is better.. If you can fit the 125, look into a 180 as well. I love the dimensions of mine, 2 foot wide gives you a lot to work with.
They can be had for cost, look around. I got mine for 800 bucks reef ready. Take your time.. In this hobby rush turns to regret.
-KC