I have used a variety of containers for seahorse fry over the years.
I started with small aquariums but had best luck using 4L wide mouth pickle jars (12 of them) with open ended air lines for circulation, light 24 hrs a day, and greenwater to consume any ammonia produced in between the water changes every 48 hours. I used them until they were large enough to transfer to larger aquariums, usually about 6 to 8 weeks once weened onto frozen.
Eventually I moved to a muck bucket system with sump which would be a small scale version of what Dan Underwood had been recommending some time back.
The muck bucket system worked much better for me and I had fare less loss getting them to the 8-10 week stage where I transferred them to a 40g breeder tank. From the breeder they were moved to a 90g tank as they grew.