I’m thinking that your mangrove and seahorse setup is too shallow to make a good habitat for large seahorses such as Hippocampus erectus, which would be vulnerable to problems with gas bubble syndrome in such a setup.
Likewise, if you cannot treat the tank with fenbendazole or Panacur to inhibit the growth of hydroids, it would become problematic to keep a colony of dwarf seahorses in the tank.
However, MJ, a long shallow tank such as yours would be a good habitat for pipefish. You may wish to consider keeping dwarf pipes, or banded pipefish, or Gulf pipefish in such a setup.
I will send you a couple of documents off list that you may find helpful for your project. The first of these documents is devoted to pipefish whereas the second document discusses compatible tankmates for seahorses, and may give you some ideas regarding this sort of inhabitants that might do well in a tank like yours, MJ. Please feel free to download the documents, save them on your computer, and then read through them at your convenience.
Best wishes with all your fishes, MJ!
Pete Giwojna, Ocean Rider Tech Support