I agree with Suzanne — I don’t think you’re necessarily doing anything wrong at all. Quite the contrary, you must be doing a lot of things right when it comes to rearing your fry. It is very common — almost the rule — for home hobbyists to lose the entire brood during their first few attempts at rearing. You have managed to raise almost 1/12 of your brood well into the juvenile stage, and four of those youngsters are almost at the five month old mark. And it sounds like your results would have been even better if it hadn’t been for that outbreak of hydroids. That is not bad at all for a home breeder!
So don’t be discouraged. As you refine your methods and become more proficient at providing suitable live foods for the newborns and work out the feeding regimen that’s most efficient for your particular circumstances, your results will get better. One suggestion I can make in that regard is to try providing your newborns with copepods for the first week or so to get them off to a good head start, as discussed below.
Copepods are the ideal first food for H. erectus and other pelagic seahorse fry. Research indicates that newborn H. erectus fed with live copepods for the first 4-5 days of life have markedly increased survival rates compared to newborns that receive Artemia nauplii as their first foods (Gardner, 2002). This is because the copepods have a much superior nutritional profile with much higher levels of highly unsaturated fatty acids and are a natural prey item that seahorse fry can digest much more easily than Artemia (Gardner, 2002; Payne, 2000; James, 1998). Studies indicate that copepods comprise a large percentage of the diet of seahorse fry in the wild, and they are therefore very well adapted for digesting such prey. Newborns have a much more difficult time digesting Artemia (Warland, pers. com.), which is removed from the marine environment and ocean food chains by several million years of evolution (Gardner, 2002).
The problem with relying on copepods as the first food for seahorses is that the pods are much more difficult to culture and provide it large quantities then Artemia nauplii. Fortunately, this is not an insurmountable obstacle, since they only need to be provided for the first four or five days of the newborns life. The superior nutrition provided by the copepods gets the newborns off to a great head start and allows time for their delicate gastrointestinal tract to develop and mature. As a result, after four or five days of growth and development feeding on lipid-rich copepods, the H. erectus fry are then able to digest Artemia much more efficiently and can thrive on a staple diet of newly-hatched brine from there on in.
You are correct that the older the fry become, the better their chances generally are and the lower their mortality rates tend to be. Survivorship improves significantly once the newborns make it past the high-risk pelagic phase of life, and there is another market increase in survivorship once the juveniles are successfully weaned onto frozen foods. It’s unusual for juveniles the age of yours to develop buoyancy problems, which are most often due to the fry accidentally ingesting air. You could try pressurizing the two juveniles that tend to float sideways at the top in a flow-through container at a depth of about 40 inches. The increased hydrostatic pressure at that depth may help restore them to neutral buoyancy again.
If you look up Don’s message titled "Sick seahorse: floating" on this forum, you’ll find complete instructions for pressurizing a seahorse in a homemade decompression chamber in case you want to give it a try, Carrie. You can find the discussion at the following URL:
Best of luck raising your juveniles to maturity, Carrie!