As long as your aquarium parameters are good and your Sunburst was eating the red volcano shrimp well when he arrived, I wouldn’t be overly concerned about its apparent lack of eating at this point. It sometimes takes new arrivals a couple of weeks before they settle in and resume their normal feeding habits as they start to feel more at home in their strange, new surroundings.
If the new Sunburst does not look thin — i.e., he doesn’t have a sunken, pinched-in belly or concave abdominal plates — he must be getting something to eat even if you haven’t actually observed him feeding on the frozen Mysis yet. The finicky Sunburst may be grazing on natural fodder off and on, or feeding on the sly and snatching up frozen Mysis when he thinks you’re not looking, or possibly even feeding after dark (it’s been reported that Hippocampus comes and H. ingens in some areas have adopted nocturnal feeding habits in the face of heavy fishing pressure during the day).
However, if the Sunburst is starting to lose condition and look skinny, then I would be concerned that he has been with you for two weeks and is apparently not eating anything at all at this time. To determine if a seahorse is losing weight or underfed, you need to examine its girth rather than its profile. All seahorses have well-rounded abdomens if you look at them from the side view or profile, even if they’re quite emaciated. To see if its belly is full, you need to look at the seahorse from the back or the front. If a seahorse is getting enough to eat, it’s abdomen or belly rings should bulge out slightly at the sides, giving it a slightly convex cross-section when viewed from head on. An underfed seahorse that’s not getting enough to eat will have a pinched in abdomen, which is sunken in slightly from side to side, giving it a concave cross-section when viewed head-on. In other words, the abdominal plates of a well-fed seahorse should appear like "( )" or at least be flush rather than ") (" when viewed from the front or the back.
(We never want to see sunken, severely pinched-in abdomens on our seahorses! Concave belly rings are a sure sign of an underfed seahorse, with the sole exception of a female that has just transferred her eggs.)
It’s a good sign that the new Sunburst is active and curious about the snails and his surroundings. When first introduced to a new tank, seahorses are sometimes shy and reclusive. This is because they normally rely on concealment and camouflage for their safety and well being, and until they are assured their new surroundings are safe — free of predators and other dangers — they are sometimes reluctant to move around very much. Movement blows their cover — breaks the illusion that they are an inanimate object and ruins their disguise — so when they feel insecure, they tend to hunker down and stay put. The activity and curiosity your Sunburst has shown indicates that he is starting the settle down and feel at home in your aquarium, so hopeful it will be much longer before he’s eating the frozen Mysis freely again as well.
It could be that they finicky Sunburst is still a little leery of his new tankmates and that that’s why he’s reluctant to come on at feeding time and eat openly. The Cardinal fish and purple firefish usually make great tankmates for seahorses, since they are gentle, inoffensive fish that won’t outcompete them for food, but skunk clownfish can sometimes be a little territorial and aggressive towards newcomers. (Clownfish in general can be a little territorial at times, perhaps due to their need to defend the anemone in which they live all their lives in the wild.) If he feels like he’s intruding on the other fish’s turf, he may not be relaxed enough yet to come out and feed openly in the other fish’s presence. That should change as he becomes more accustomed to his new tankmates and new environment. At the seahorse ranch in Hawaii, he was living in an open system with pristine water quality, natural seawater, natural sunlight and plenty of room to roam. Now he’s making the transition to a small, closed-system aquarium with artificial saltwater and artificial lighting, and a group of new tankmates he’s never encountered before, and it may naturally take him a little while to adjust to the different conditions.
The presence of the other Sunburst who appears to be very much at home and is already eating the frozen Mysis very well should help him make that difficult transition. But for now, concentrate on feeding the other fishes first and then continue to try gently and patiently target feeding the finicky Sunburst. While you are feeding the seahorses, try to get a good look at the reluctant Sunburst’s midsections as unobtrusively as possible. I want you to update me on the Sunburst’s behavior again after he has been with you for a few more days. Be sure to report whether the seahorse appears to be losing weight or developing a sunken abdomen, or if he seems about the same. When a well-fed seahorse is observed head-on, its abdomen should appear smooth or slightly convex in cross-section, not concave.
If not Sunburst starts to look skinny at all, with that pinched-in stomach, try tempting him with live foods again. I would recommend adult brine shrimp, which are inexpensive and readily available from many local fish stores, as a stopgap measure. They can be enriched to increase their nutritional profile and should keep him going until he has adjusted well enough to resume feeding on frozen Mysis again. If all goes well, that shouldn’t take long, since he should soon follow the example of the other Sunburst that is eating Mysis greedily.
Best of luck with your new Sunbursts, Joanie!