I'm coming into this thread a little late, but I wanted to comment on the article at seahorse.org that you linked to.
First and formost, I strongly dissagree with their stocking levels and the reasons why.
I do not think anyone should keep even two of a small species like H. fuscus in a 10 gallon tank. While it can be done, the issues with maintaining good water quality for your horses and the resulting risk of disease are just too great for most people.
Next, while seahorses are, relatively speaking, poor swimmers, even a 90 gallon tank is small when compared to the territories they keep in their natural habitat. For the few species where territory studies have been done, they average territories on the range of 6 to 10 metres square. Thats the size of a large livingroom to the size of a small house. How many 90 gallon tanks can you fit into a space this size?
To suggest that a creature which, in nature, maintains a territory much, much larger than the average seahorse tank would get worn out and stop looking for food in your tank is just plain silly. It shows a lack of knowlege of the basic biology and behavior of these animals.
As a secondary point, there is no reference to any research done to prove the point being asserted. That makes it nothing more than idle speculation!
If you feed your horses regularly they will find the food and eat it. From watching my seahorses for several years now, I suspect that they form, keep and update a mental map of the tank and know where the food is likely to be. They regularly hang out and hunt in the area where my refugium empties into the tank. If I rinse pods from my refugium algae into a particular area in the tank, the horses will return there the next day to continue hunting. To repeat, they know where the food is likely to be.
OK, I'm done with my rant now.
To answer your question, I think you would be fine to start with a single pair. Just remember, to minimize disease problems, if you want to add more horses later on, they should be from the same souce as the original horses.
If you are concerned with how to feed them in such a large tank, section off a small part of the tank to establish it as a feeding area. Once they have been feeding in the small area for a few days, let them roam the rest of the tank.
I wish I had room for a larger tank. My dream seahorse tank is a 130 gallon setup with seagrasses on one side and rocky rubble at the other end.
Good luck.
Fred
