I've had one of these fish thriving in my tank for years. In fact, I think it has pretty much reached full size. I think the main reason for this is that it eats flake food.
The problem I see with this fish is that it goes after food aggressively, but is highly inaccurate with its beak. It's almost comical to watch it stabbing away at floating food and missing 90% of the time. Generally, it is only successul in targeting the food once it has sunk to the bottom or become lodged in a rock. This species evolved into specialized feeders that are able to probe very small holes and crevices for food items that other fish can't get to. If it is surrounded by "fat" aggresive tankmates, not much food is going to find its way into these places. You were probably right about the fish doing better on it's own.
While the shrimp might be a more natural diet, flake food has a much higher concentration of protien and nutrients. Therefore, the fish doesn't have to consume as much to be healthy. Certain brands of flake food are packaged as huge flakes, so if the fish can grab just one of those, it's going to get a pretty good sized serving. Plus the big flakes are much easier to target. Their snouts are not designed to eat big chunks of shrimp, but the flakes get so soft and flexible after they are soaked, they can suck of big pieces, kind of like they're using a straw.
Ocean Nutrition is my preferred brand. It is vitamin enriched, has a high percentage of protien, and a wide variety of natural ingredients. I alternate between Formula 1, Formula 2, and Prime Reef. Each has a slightly different composition and my Yellow Longnose loves all 3. The bigger container you buy, the bigger the flakes you get, so shop accordingly. Do not break up the flakes up before putting them in the tank. Even small fish can pick them apart if need be. Always re-stock before the container gets to the bottom where the flakes are all crushed up.
The "don't feed them more than they can eat in 3 minutes" rule should probably go out out the window as well. Obviously you shouldn't go too far overbaord, but you also need to keep in mind that the specialized beak can keep picking up leftovers long after the other fish have lost interest. Mine is constantly pecking and grazing, even at night when the rest of the fish are dormant. For that reason, it's also not a bad idea to throw in a small pinch after lights out.
Another suggestion I would make is to stop treating your fish for parisites unless you're pretty sure they have parasites. Anything that kills parasites can't be good for other forms of life either. That type of fish has clear fins, so any external parasites are going to be obvious. And if it's still feeding aggressively, it probably doesn't have any internal bugs either. A healthy, well-fed fish can usually shake off most parisites, and over time, develop some resistance to them. I've seen a few white specks on my fish from time to time, but I never worry about it too much, and so far it's always cleared up fairly quickly. The Yellow even had a pretty good sized isopod or something attached to its fin a while back, but that thing had dissappeared by the next day.
Quarrantine doesn't mean you have to soak your fish in a bath of chemicals. The main benefit is isolation from stress and competition while acclimating or recovering. If a fish is feeding well and shows no signs of disease, there is no reason to treat it for anything. Besides all that, I've found that whenever a fish develops a really serious internal disease, no amount of treatment is going to save it.
If your fish is still feeding agressively, then I don't think you have too much to worry about. All you have to do is figure out a way to get it enough nourishment. That fish gets pretty big, so it will need to consume a massive amount of those baby shrimp in order to grow properly and maintain its weight. I would put it back in quarrantine until it starts to fatten up a little. If you have decent filtration, and dont treat the water this time, you should be able to go at least 2 weeks without changing the water. Use water from your main tank, and you shouldn't have to worry about it cycling. That way you can experiment with different foods until you find something that provides the best odds of keeping it well fed amongst its tankmates. If it won't eat flakes, there are other options.
I hope this helps. Please reply if you have further questions.
