Well, your existing tank certainly has good dimensions for a seahorse setup, sir, if it’s 30 inches long and 22 inches in height.
Whether or not the Koralia Nano powerhead will provide adequate circulation for the whole tank from within is probably going to depend on how you have the rock formations or décor arranged. You may need to use two of the Koralia Nano powerheads, one at each end of the tank, in order to make sure that there was good circulation throughout, so you might have to do a little experimenting to see whether or not one of them will suffice.
Even if you ultimately end up with two of the Nanos, Don, that may be preferable to using a Koralia 1, which would likely be a bit too overpowering for a tank that size. However, if there are formations of live rock to deflect and diffuse the output from a Koralia 1, then it may also work well for your particular tank. The only way to tell for sure is to try them out and see which one provides the best overall water circulation without overwhelming the limited swimming ability of your seahorses.
Best of luck achieving the ideal flow patterns for your particular setup, sir.
Pete Giwojna, Ocean Rider Tech Support