Seahorses
Note: Many species of seahorse are threatened. Aquarists have a responsibility not to add to the pressure on wild seahorse populations. you want to know more, visit Project Seahorse.
None of the traded species of seahorse are found in brackish waters. There are some species that do occur in estuaries, notably the North Atlantic species Hippocampus erectus and the endangered Cape seahorse Hippocampus capensis from South Africa, but neither of these is likely to be found in the tanks at your local tropical fish retailer.
All seahorses need a quiet tank with lots of live food. There is really no reason to keep these fish in anything other than a marine tank by themselves or with very quiet fishes such as neon gobies or mandarinfish. Kept properly, they are actually quite robust and tolerant, as well as exceptionally beautiful.