That depends on the type of seahorses. For example, a 8-gallon aquarium is large enough to support an entire colony of dwarf seahorses (Hippocampus zosterae) and all of their offspring, and that is certainly the species that is best suited for such a small tank. But an 8-gallon tank is way too cramped for any of the large seahorses, such as Mustangs and Sunbursts (H. erectus). They would be unable to mate successfully in such a shallow aquarium and the lack of water depth would leave them vulnerable to problems with gas bubble syndrome.
Aside from the dwarfs, which are ideally suited for small aquaria but which you don’t appear to be interested in, you’re only other option would be to consider a single pair of the smaller Shetland pony class of seahorses instead. The "Shetland pony" category include Zulu-lulus or Cape Seahorses (Hippocampus capensis) and the short-headed seahorse (H. brevirostris), which are temperate seahorses that require an aquarium chiller to maintain suitable temperatures, or the Black Seapony (H. fuscus), which is a tropical species that would do well at standard aquarium temperatures.
All of these species may be small enough to try in a 8-gallon aquarium with efficient filtration, and like the dwarf seahorses, they are considered among the easiest of all seahorses to breed and raise. Please contact me off list ([email protected]) and let me know if you would be interested in keeping either of these species, and I will be happy to send you a species summary that explains everything you need to know about them in considerable detail. Or you could search for more information regarding the dwarfs, the Zulu-lulus, or the H. fuscus on this forum and find a great deal of additional information about them right here.
Best of luck converting your nano tank to accommodate seahorses, Jill.