Yes, with a tall vertically oriented aquarium like your 55-gallon hexagonal tank, it is very important to establish good water circulation from the top of the tank to the bottom in order to assure that there is good oxygenation and aeration throughout the aquarium.
Using a wave maker is a good thought, but such devices are most effective when used with rectangular horizontally oriented aquariums and are less well suited for a hex tank.
In your case, Desa, you might be better off installing a small power at the top of the tank that is positioned so that it directs a water current diagonally downwards from the top of the tank to the bottom of the aquarium on the opposite side. That would serve to establish a good circular water flow from the top to the bottom of the aquarium more efficiently and inexpensively than a wave maker for a hexagonal tank like yours.
As far as the stocking density is concerned, your 55-gallon hex tank can safely accommodate a whole herd of hardy captive-bred-and-raised seahorses, Desa. But it’s always best to keep your aquarium under stocked in order to provide a comfortable margin for error, especially if you’re new to seahorse keeping.
I recommend starting out with one pair of ponies and then gradually increasing your herd, rather than adding multiple pairs of ponies at one time. Having too many new specimens at once can overtax the biological filtration temporarily, resulting in transitory spikes in the ammonia and/or nitrite levels that could be detrimental to the seahorses and other aquarium inhabitants.
But over time, a 55-gallon hex tank can safely accommodate several pairs of adult seahorses and still retain a comfortable margin for error. Just build up your herd gradually, one pair at a time, for best results, and everything should go just fine.
Be sure to contact me whenever you have any more questions or concerns, Desa.
Let me know when your biological filtration is fully established in the tank is there a chance to mature and stabilize, so that you are ready to order your first seahorses.
Best of luck with the final preparations for your 55-gallon hexagonal aquarium, Desa!
Pete Giwojna, Ocean Rider Tech Support