If you don't already have experience with keeping seahorses, you can check out the links at the bottom of "
My Thoughts on Seahorse Keeping".
They are written by experienced seahorse keepers from the "org" plus one important article by Dan Underwood of seahorsesource.com.
You will find that a 30g tank is sufficient for one pair of standard sized seahorses like reidi, erectus, kuda for example.