I have had a 6 foot 125 for a few years now, and I have had to move quite a bit of livestock in and out of the tank due to making some bad decissions as to which fish are appropriate for this sized system.
Lisa stated things well. You can keep triggers in this system, but if you plan on a community seting then you are very limited in terms of which triggers you can keep. A huma will not work in this volume unless you house him/her with only a couple of very aggressive tankmates. Otherwise, the huma upon reaching sexual maturity will beat up all of his/her other tankmates (experienced this on 3 separate occcassions until I finally accepted that this was the reality). In terms of triggers for a community tank for a 125, your choices are pretty much a saragassum, blue throat, or Indian. Every other species gets too large and/or aggressive for a community seting in a standard 125.
To give you some idea of what a fully stocked 125 can house (am fully and really a little overstocked), this is my stock list: indian trigger, yellow belly dogface puffer, blue dot toby puffer, one spot foxface, pinkface wrasse, Australian tusk, flame hawk, and yellow tail damsel. Most of these fish have lived together peacefully now for a couple of years, except the indian trigger has only been in the system about 8 months. They are all full grown or just approaching full grown size. For example, my dogface is about 7-8 inches, the Indian is about 7 inches, the tusk is about 6.5 inches. These three have a bit of room to grow. My one spot is full grown at 7 inches, and the pinkface, toby puffer, yellow tail damsel, and flame hawk are pretty much full grown also.
There is some minor aggression in my system, but nothing significant. The yellow tail damsel will take a shot at the toby puffer from time to time when the toby gets too close to the damsel's cave. Likewise, no love is lost between the tusk and the pinkface. They pretty much stay away from each other and do not fight. However, when they happen to bump into each other on occassion, the tusk will give a brief charge at the pinkface, but the pinkface is such a good swimmer that he easily evades the tusk and no physical contact occurs. The pinkface will in a very non-commited way give a brief charge toward the damsel or the flame hawk from time to time, but he really never makes much of an effort to do so. Suprisingly, my foxface will patrol the rocks and raise his spines at fish, including the trigger, when he feels they may be trying to eat algae from the rocks. Again, never any physical contact. Otherwise, everyone gets along very peacefully and have very little aggression in the system.
