Seahorses are social animals and do better in pairs or groups. It doesn't have to be M/F pairs, it can be all males or all females. They just want a pal to hang out with. In my years of keeping seahorses they tend to become depressed whenthey are kept alone. Very sad to see.
As for the tank size. You "could" live in a bathroom your whole life. But would you want to? If we are to keep animals in our homes it is our responsibility to make sure they are happy, healthy, and comfortable. They need room to swim, court, hunt, play, LIVE. Just like everyone and everything else.
Angie