But like i said it depends on he speices you get.
Five seahorses in 10 gallons? Not a chance. I cannot stress this point too strongly: seahorses are extremely difficult to maintain unless you understand completely their very specific needs. They are not fish you can cram into a small, low-maintenance tank. I consider myself a reasonably capable aquarist, but seahorses are, like stingrays, fish I wouldn't keep because I don't have the time or the money to maintain properly.
But like i said it depends on he speices you get.
The species in the trade vary from quite small dwarf species up to specimens around 30 cm long. So yes, the size of the tank required does depend on the species you want to keep. Put another way: identify the species on sale in your local shop, find out its Latin name, look up its size requirements, check what types of food it accepts, and then decide whether you can keep it. Many of the smaller species (like
Hippocampus zosterae) are strictly live food only, and it is the bigger species that tend to take frozen foods.
Cheers, Neale