Reidi is a great first choice. It was my first seahorse, got me into this hobby. I was able to keep him for 8 years.
This is unpopular but I would not get a seahorse from an LFS unless the LFS had a separate system setup for seahorses and was keeping them below 73F. It's a black or white thing too, 74F bad, 73F good. The temperature issue has to do with a specific bacteria that we believe is part of the seahorses digestive system but not confirmed yet. Anyone these species of vibrio actually change their protein structure at higher temperatures and become much more virulent. The seahorses have no immunity after the protein change. There is a book called "Working Notes" by Dr. Belli that explains it much more clearly. I believe Noga wrote about it as well, but since I know Dr. Belli I gotta recommend him first
Reidi is a bit harder to breed than erectucs, as the first couple of weeks Reidi fry require rotifers. If you are just starting out it is not really a concern. Just get all females and make your life easier. Also skirts any complications that can arrive with pouch issues that males can get.
The very best way for water to move in a seahorse tank is with a spraybar that is under the rocks. You can push a lot of water through the spraybar that way and it will GREATLY help with algae and detritus accumulation. Seahorses are very inefficient eaters, and they require a lot of food, so having the flow come out through the rocks keeps food/poop from accumulating under the rocks and just solves so many issues.
Here is a not very good picture of how I ran mine. The tank was acyrlic so I was able to drill through the back and run a closed loop through spraybars. I had two, one on each side. I painted them purple so they would blend in with the rocks. I had about 8 feet of pipe in that tank, that no one every saw once the rocks went on top of it.
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Here is the tank when it was newly setup before the macro algae grew in:
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Few months later:
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Ended up around here:
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If you can find it I would really recommend macro algaes as well. Try to get one fast grower like a prolifera, (just pinch the runners every few inches every so often). Macro's can suck up a ton of nutrients and you are going to need that extra support.
Generally green macro grows fastest, red is usually slow, and comaparitively blue or purple is slowest.
I also think that some types of LPS, blasto's, acans, make great tankmates as they help to break down the uneaten foods. Leathers are also very good as they suck up nutrients as well.
Steer clear from most fast eating fish. A lot of people say a fish will be OK with seahorses but if you observe the seahorse without the fist they act differently in the tank. Seahorses don't need a species only system, but they like things pretty chill. I'd focus on some fish that will do some work. Fish that sift the sand, some fancy peppermint shrimp (
Lysmata jundalini ) as long as they are over 1" are great tank mates. So many gobies are great.
Look up a feeding station, and get your seahorses onto that as quickly as possible. I just used a shell glued to a magfloat. It makes clean up a lot easier and will save you some hassle.
Also if you want to run a protein skimmer, ancedotal evidence IME has shown that it needs to be run in the sump, and not a HOB style skimmer that is popular. If you have an AIO tank and the skimmer is emptying to the back portion and then to the tank it is OK, but having a skimmer emptying directly into the tank with male seahorses specifically has a much higher rate of Gas Bubble Disease IME. Since the medication to treat GBD is now illegal in the US, it's easier to just have the skimmer in the sump, or empty to the back chamber.
Seahorse.org has a great article on it with pictures.
If you need any help, please reach out.