From just below the surface to something around 3 feet. Pallets, boards or waxed cardboard floating on the surface (preferably stationary). 1 to 2 ft deep is what I would shoot for..
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+1 IMHO, in the absence of predators, floating or suspended spawning surfaces work well. If the pond has BG or other predators, I would stick with sunken habitat in shallow water, like pallets, where predators have very limited access to the spawning surfaces and new spawn from below. Nearby vegetation for the new spawn to hide and feed in is also a plus if the goal is to maximize survival of the FHM.