I have one, I have had it for about 5 years. Let me start by saying its a really cool fish, given the proper tank mates, but VERY shy. My betta is about 8" long, bought at 2", but a good ammount of that is fin so its not the bioload that you would expect, more like 5" body 3"+ fin, at max size.
http://reefkeeping.com/issues/2002-04/hcs3/index.php
Just a few highlights of the article that I can add my personal experience to:
They are slow feeders, often giving their prey a long stare prior to the attack.
That is what makes them incompatible with many of the more commonly kept fish, they will have food stolen from them before they have had a propper ammount of time to 'scare' their food. Yes they try and scare dead krill, mine will even circle the krill once or twice before slurping it down, even now.
The second defense, known as Batesian mimicry, is instinctive behavior that can occasionally be viewed in our tanks. The Comet will dive head first into cover at the first sight of danger. Rather than bury itself and completely hide, the fish will leave his back end viewable. To the potential attacker this looks similar to a Gymnothorax meleagris, the white-mouth moray eel, and usually results in the attacker withdrawing (Lieske, 1999).
Mine will do this often, really if I come up to the tank, and the betta is out and around, more often than not it will.
Prior to purchase ensure you have the proper environment for your Comet. The tank should have numerous crevices which are devoid of light. A cave or an overhang is highly recommended. Brightly lit tanks are not the best option, but as long as dark corners exist your Comet will be fine.
Mine comes out with all 6 T5s on, but still defiantly needs a good cave.
Getting a Comet acclimated to your tank conditions will be the biggest hurdle you'll have to overcome. They acclimate slowly to the aquarium. Once added, they will generally dive deep into your rockwork for up to a week or more. If you want to see your Comet more often during daylight hours, setup a cave or overhang that is devoid of direct light. Choose one corner or the other, not the center of the tank. This should avoid a good percentage of traffic from swimming fish. Avoid any direct light. Light reflected from the sandbed should also be avoided. However, make sure this cave is readily viewable, as you will initially need to sneak up to the tank to observe your Comet. Actinic lights are another great addition for additional viewing of the Comet. With a dawn/dusk photoperiod you'll get to witness the Comet in open water on a timed schedule. They tend to appreciate the softer light that actinics provide, and reward you with extended viewing of your fish.
The actinic lights really make the white spots stand out, really cool looking fish.
Getting your new Comet to accept prepared foods is another tricky part of successfully maintaining this fish. I suggest trying prepared foods first, not live foods. It's probably best that your Comet not become accustomed to live foods unless it becomes necessary to induce a feeding response. Since Comets are night predators, night feeding might be necessary. Feeding during your tank's actinic phase is perfect. Try frozen/thawed mysis or plankton as your first offering. Most fish find this food irresistible. Small frozen/thawed krill or silversides would be another choice, followed by brine shrimp. It may take your Comet a week or more before it takes it's first food. I wouldn't concern myself until two weeks have passed with no feeding observed. Comets have a rather slow metabolism. After two weeks without a confirmed meal, I'd switch to live foods to entice feeding. Live freshwater mollies, ghost or grass shrimp, black worms, or brine shrimp should be fed. I suppose live peppermint shrimp could work as well. Eventually, you'd like to get the Comet weaned towards prepared foods. Long-term diets should include Mysis, plankton, Formula 1™, krill, and silversides.
Mine was rather skinny and only 2" long when I got it, so I fattened it up on gut packed ghost shrimp, it was eating mysis the next day. Now that its full size I feed frozen cubes and enriched krill.
Good tank mates for a Comet would include small wrasses, dwarf angels, grammas, jawfish, larger gobies and blennies, cardinals, and hawkfish, to name a few.
Bettas are VERY shy and in my experience will never fight for food, my six line wrasse would sometimes try to steal food from the bettas mouth. I would avoid any fish that are fast gobbling eaters. Dwarf Lions are a match made in heaven with these fish!!! They share so many traits and are so perfectly compatible that I would think it crazy not to keep them together!
it just really struck my curiousoty when i saw one cause it looks somewhat like a fw betta, but oh so different lol. really cool fish IMO
thats not it... but its very pretty! looks somewhat similar to it, but this one is SUPER vibrant and has dark purple and green in the background.... its definately not a goby... do you need a sump for a 55gal? and whats a sump? im TOTALLY ignorant when it comes to SW set ups 
i kno u dont need one for like a 30gal... but whats the cut off to where you need one and whats its purpose?
