If I were you, i would assemble your rocks and powerheads so that there is a small area w/ higher flow (enough for SPS), and then the rest of the tank be sort of slack water.... The seahorses do not mind very strong water currents, as long as they have some where to retreat to that has slower water flow. This would allow you to keep SPS, and other corals that require high water flows without harming your horses at all. I have a 29 gallon tank where one half is high flow, and the other is slackwater. My horses seem to have fun getting whirled about, and then retreating to the safety of a hitching post.