I can tell you what I did.... but it's not the perfect setup. Keep in mind I had 7 tanks (reefs, FO, freshwater) running in addition to seahorse fry - so space/time is limited.
All I use for filtration are Hydro sponge filters. I use these in the fry tanks, the juvie tanks AND I have 4 of them in the barebottom adult tank. That's it....no sand, no rock, no nothin. Just barebottom, hitches, sponge filters. (I do have a display tank at work with sand, live rock, etc that houses 3 second generation and 2 third generation Erectus that I raised)
As far as the fry.... when they are born I put them all in a 10 gal tank with 2 Hydro sponge filters. Unlike Dan, who is THE most generous/helpful person I've ever had the pleasure of speaking with and dealing with, I use much smaller accomodations for the fry. Mainly because I don't have the funds/space to do anything bigger.
I did try using 5 gal tanks for the first few days... but the die-off was incredible due to overcrowding and the quickly deteriorating water quality. While 10 gal isn't perfect, it was big enough for me to control the water quality and give them a bit of space to move around but was small enough that I could keep my eye on the tank to see who was eating and who was not.
Anyway, the first week to 10 days they are in the 10 gal and I feed as often as I can. There will be die offs during the first week or two - usually those that aren't eating well, are weak, or may be "floaters". Around the 10-14 day mark, I'll pick a day and feed pretty heavily. Those that go after food eagerly, seem strong, etc are considered "the keepers". Any that don't eat well, can't eat for whatever reason, float to the top, lay on the bottom, get blown around and don't swim well, seem to be "clear" in color instead of solid color....well, those are the "non-keepers". There are many factors...but those are some of the main ones I considered.
After they are seperated, the "keepers" were moved to a 15 gal tank. Usually it would number upwards of 50. Again, two sponge filters and hitches were all that were in the tank. It was siphoned at least 3 times a day. Fed as often as I could. From there I would split the group into two 15 gallon tanks after they grew a bit, and after that, moved them to 20 gal tanks. After that, I combined them all in a 45 gal until I could find homes for them. Again, space was cramped, which wasn't the best scenario, but it's what I had to work with.
I'd guess you're right.... of "the keepers", I'd say I usually ended up with between 25-50% survival. Depending on the time I had to spend with them. Even at 25%, that's still about 15 seahorses. At 50%, that's about 25. As you can see, if you even only get 25% survival rates, you could end up with 100 seahorses needing homes in the course of several months. Especially if you have multiple pairs as I do.
NOW you see why I stopped. Even at a 10% survival, I couldn't keep setting up tanks waiting for people to buy/take the juviniles locally. Don't get me wrong - I never had 100's of seahorses. But I did have 3 or 4 pairs breeding so there were times I had 30 or 40 at different ages.
Breeding/raising the fry is a fun, enjoyable challenge I wouldn't trade for the anything. At this time, however, I have no plans on putting the pairs back together to breed. I still have 16 adults.... and one young adult I kept from the last brood I allowed them to have. Funny thing about that one young adult. Of ALL the fry/juvies/adults I have, every single one of them is/was a yellowish/orange color. Every one of them. EXCEPT this one young adult. It's the only one I've ever had that was, and remains, solid black with silver saddling. That's the only reason I kept her.
Best of luck!
