You’re very welcome, sir!
When transferring the fry, scoop them up in a small measuring cup or something similar along with a little water. It’s important that the newborns aren’t exposed to the air during the transfers. Or a plastic turkey baster works well for delicately sucking up the fry when transferring them, providing you cut off a bit from the tip of the baster to enlarge the opening at the tip. Just don’t net them out.
The recommended stocking density for pelagic seahorse fry such as H. erectus is no more than 25 per liter, or a maximum of 80 fry per gallon. But for best results, I would recommend that you limit yourself to no more than 6 fry per liter, or a maximum stocking density of about 25 fry per gallon, while you are learning the ropes and gaining experience at rearing.
I know of no ready made, off-the-shelf refugia or crater keepers that work well for in-tank nurseries as is, without major modifications. At the moment, fabricating suitable in-tank nurseries is very much a do-it-yourself project. Most in-tank nurseries are modified critter keepers or plastic goldfish bowls that have been adapted as nurseries by cutting a large hole (or a series of smaller holes) at one end and then screening of those openings with fine mesh (e.g., 400 micron screen). Water from the host tank is poured into the in-tank nursery several times a day in order to provide the fry with frequent water changes.
Modified plastic critter keepers or critter carriers are perhaps the most often used containers to create suitable in-in-tank nurseries, so that might be a good place for you to start, ageber. There is still room for a great deal of experimentation in order to determine the optimum design, so don’t hesitate to get creative. But I would avoid breeder nets — they tend to get quite a bit dirtier than bare-bottomed nurseries (uneaten brine shrimp and fecal pellets will accummulate on the netting and cling to the mesh) and are too difficult to sterilize and keep sanitary.
I should think either your 90-gallon tank or the 50 gallon refugium (or both) would make good host tanks for housing a number of in-tank nurseries.
Best of luck with your first attempt at rearing, sir!