First of all, the recommendations for keeping seahorses is 29g min for the first pair, and and extra 15g for each additional pair.
Next, you need to be able to keep the tank cool in the range of 68° to 74°F due to seahorse propensity to succumb to bacterial infection.
Seahorses are also known to be prone to infestation problems from parasites they have not grown up with, even those of other seahorses not raised in the same facility.
That means there is a good chance they can have problems with parasites already in your system from other fish.
As for purchasing seahorses, most sold in stores are tank raised using ocean water and as such are not any better than wild caught other than they are usually trained to eat frozen foods.
Wild and tank raised, to give best chance of success in their keeping need to be put through a deworming process of about 9 weeks with 3 different medications.
The procedure can be found on seahorse.org as well as a much more information in the library there and in the various forums on that site.
Your better chances of success will come from buying seahorses raised by another hobbyist or from buying from places like seahorsesource.com or seahorsecorral.com.
