Congratulations on raising so many of your juvenile Hippocampus erectus to the 12-week mark — that’s quite an accomplishment for a home hobbyist, indeed! It’s wonderful to hear that they have all made the transition from newly hatched Artemia nauplii to frozen Mysis so smoothly, since it’s quite common to lose some of the youngsters while weaning them from live foods on to frozen fare. Well done!
Minced Mysis certainly can be a messy food and you have to be more diligent than ever about maintaining water quality in your rearing tanks so that any of the smoky residue and uneaten tidbits don’t degrade the water quality or cause transient ammonia spikes. But it’s a landmark event whenever you get your latest brood successfully weaned onto frozen Mysis — it’s such a nutritious, easily digested natural food for seahorses then it’s well worth the extra effort. Your seahorses are still growing rapidly at this age and need all the calories they can get, so don’t hesitate to allow them to eat their fill of the frozen Mysis. Just be sure to stay atop of the water quality while you do so.
If you can equip your 29-gallon grow out tank with a small protein skimmer or a surface extraction box for the filter, that would effectively remove and prevent the oily film that the frozen Mysis tends to produce, and make it easier to maintain optimum water quality.
Now (i.e., after they have been weaned onto frozen Mysis) is the time when you can start thinking about transferring some of the juveniles into the main tank. They need to be eating the frozen Mysis well and they need to be large enough to handle the stronger currents in the main tank, but it sounds like your 12-week juveniles have just about reached that point, Tammy. I would say that you could start moving a few of the largest juveniles into the main tank with the adults whenever you are comfortable doing so. Just keep a close eye on them at first to make sure they are able to handle the currents in the main tank and are able to get their fair share at feeding time (you may need to target feed the juvies individually at first to make sure that they are not being bullied or outcompeted at mealtime).
As you know, Tammy, one of the neat things about raising Ocean Rider’s Hippocampus erectus is the tremendous amount of genetic diversity that is built right into the strains. This will often manifest itself in variable coloration among the offspring, and it sounds like you have one of those "rainbow" broods. They should make a very colorful herd as they grow up.
When you consider that Ocean Rider has been working with Hippocampus erectus since 1998, producing dozens of generations of Mustangs and Sunbursts, each more genetically diverse than its predecessors, it’s easy to understand why there is so much variation within a given brood of seahorse fry. It is very commonplace for brothers and sisters in the same brood to express differences in variable traits such as coloration, spininess, the size and shape of the coronet, the presence or absence of cirri, and so on.
In short, within the same brood of young, it is quite common to have some juveniles that are very spiny along with others that are very smooth skinned. In the same brood, there may be some fry with all well-developed coronets and others with only low, rounded bumps, and many whose crowns are somewhere in between those extremes. There may be a few fry with very elaborate cirri adorning their heads and necks, whereas most of their brothers and sisters have no cirri at all. The same variability in coloration holds true as well.
So you may well have juveniles that show all of the sunset colors (yellow, gold, orange, peach, salmon, certain shades of red or purple, as well as the usual brown and black, all in the same brood. It’s all a function of the tremendous diversity that’s built right into the genes of Mustangs and Sunbursts as intraspecific hybrids.
Best of luck raising your rainbow youngsters to maturity, Tammy! Keep up the great work!