Choosing the right size tank will depend on what species you want to keep, how many seahorses and whether you wish to breed them. A minimum depth of 30 cm is recommended, as tank depth is more important than width. Don't forget to check the recommended stocking densities.
Lighting: Aquariums function better with light, natural light can work well in some situations but you can enhance your tank by lighting with fluorescent tubing. It is however important to purchase special tubes that either provide daylight conditions or tubes that intensify Seahorse colours. Conditions vary if you wish to add live plants to your tanks. Fluorescent tubes specifically for marine environments are available and the choice is yours.
Aeration: All living things in your aquarium need oxygen to live. So aeration is an important requirement. Seahorses like oxygen levels to be near saturation, an airpump is necessary to provide this. Air pumps do as they suggest pump air into the tank, but there is an added bonus, as these pumps create bubbles, which flow to the surface agitating and circulating the water. Air powered filters provide aeration as well as filtration. Air stones should not be used as larger bubbles are recommended. This minimises the risk of bubbles getting into the brood pouch of Seahorses during the courtship dance.
Tank Decorations: A variety of mediums can be used as bottom substrate for your tank, gravel, sand, crushed coral/shell etc. All of these will become "live" as beneficial bacteria will cling to them during the Nitrogen cycle.
Seahorses do not spend all day swimming and will need "hitching posts". You can add live or fake plants the choice is yours. Seahorses enjoy lots of little nooks and crannies to hunt around or hitch to. Ensure there are no sharp edges or points that may damage to the seahorses fragile skin.