I too am very interested in hearing more about the temperatures at which these fish are collected.
I have mine in my 57 gallon reef tank. I have had the male for slightly over a year and finally managed to pick up a healthy female from Divers Den a week ago. The females are for some reason much harder to find and are almost always near death when I find them so I immediately jumped on the one from a reputable source like Divers Den. I am not sure what they did with her there, but she came in fatter than any Ruby I've ever seen at a retail store.
Within 72 hours I witnessed them mating for the first time, and they have every night since:
https://vimeo.com/115890389
I am absolutely fascinated with this genus of fish and have spent a lot of time on them/researching them. I have now tried 5 of these fish total. I'll try to go over a little of what I've learned.
These fish often come in mis-labeled. True Ruby's aren't truly classified yet in that they may be a new species, or maybe they are just a variant of Morrisoni, Moyeri, Tudorjensi, etc. The best way to tell true rubies is the eye spots found directly above their pectoral fins:
http://s59.photobucket.com/user/armagedon48/media/RubyRedDragonts-2_zpsdb4c75dd.jpg.html
The first fish I bought as a "Ruby Red Dragonet" turned out to be a Moyeri, which can look very similar but without the eyespot:
http://liquidlifephotography.com/photos/wp-content/uploads/2012/06/2012_26_31.jpg
I later found that they also act very differently (for a different post).
I then bought a true pair of Ruby's from an LFS, they were pretty skinny so I decided to risk it and put them straight in the display tank. As can be expected disease broke out, they eventually died along with my moyeri, and a couple other fish. Several weeks later I bought a male who looked pretty healthy at the time. He has since at least doubled in size and is the one still in my tank (one year later). He grew to this size eating only pods already in the tank. About 6 months ago he did start to eat frozen food. He gets excited when the tank is fed, and will eat food that drifts past him, however he will not actively search for or compete for the food.
I have not noticed anything to do with temperature as was previously discussed and my tank typically runs anywhere from 77-82 depending on the season. Do not expect this fish to eat prepared food. Even if you eventually get them to eat it, it will be near to impossible to sustain them on just that because they need to constantly pick. If you hope to keep them on just prepared food, I believe they would need on the order of 12-15 feedings per day, with the food targeted at them/drifting past them. The hardest part with these fish is finding an individual that is healthy enough to make it through whatever kind of quarantine you have with enough energy to start hunting and regaining weight. The pre-requisite is of course that you have a mature tank with plenty of pods/life.
Now for determining if it is healthy enough to bring home. Do the obvious checks, then make sure the fish is actively swimming around the tank/hunting. Try to look at the fish from underneath- a collection cup is ideal. When they get skinny it will show most from underneath- on either side of the belly two "ridges" will appear tracing something of a triangle of the two sides of the fish. If you can see those clearly defined, I do not suggest taking the fish home. If the belly is flat or plump, is hunting, and is showing good signs in general, go for it! Like I said, I've taken five of these fish home, however I've seen at least a dozen that I deemed too sickly to bring home and all that I followed up on had died. Quarantine is extremely hard on these fish, so for my most recent addition (the female), I simply acclimated, did a 7 hour prazi bath, followed by a 10 minute FW dip. No parasites were visible after each of these so I then added her to the tank. This is risky as it does not treat for common diseases such as crypto, but given the nature of those diseases, I decided that this was the best solution for ME. I have also done this with other touchy fish.
To answer your initial post (finally), I have never seen these fish go for live brine. Not sure why. I've tried it, but never really gotten any reaction. The "test" I suggest doing is detailed above. In a 120 gallon tank that IS MATURE, I think you would have plenty of pods for these guys even with a couple other pod munchers. They don't seem to eat A LOT, but they do seem to eat constantly. Like I said, I have my pair in a 57 with minimal LR and other pod munchers and they do very well and are FAT (well the male is, I think it's too soon to speak for the female but so far so good- especially given the mating dances!)
