i believe most seahorses need around 3x their height for mating rituals,etc...there are smaller species like zulu-lulu(H. capensis) and dwarf(H. zosterae) i would not recommend dwarves in such a large tank as adults could fit on a quarter...unless you wanted to devide the tank, however dwarves require(MUST) live brine shrimp twice daily, so you have to hatch that as well...if you look at some of my threads you'll find a dwarf seahorse thread which i have right now. the zulu lulu i am not as familiar with but i know they need a lot cooler water, between 68-72 and so it can become quite expensive needing a chiller to keep that water cool. seahorses are a lot more delicate, so i would suggest definitely doing research before purchasing....they are fun to have though, thats for sure :joy: ...hope this helped