Two to three clutches per year on your blackneck swans is great! Do you let the parents raise each brood or how do you work that? Could I ask what you're selling your blacknecks for per pair?
I think some things to consider would be the species, size of the swans, quality of the water, etc. Black swans could make due with less water and shallower (maybe as little as 2 ft deep or so) than larger species like Mutes or Trumpeters which would obviously do best with a larger pond that is...