I have some links, so please read what the seahorse requires, I would type it all, but my typing speed is slow.
http://www.seahorsesanctuary.com.au/ http://www.seahorse-australia.com.au...about_shs.html http://www.saseahorse.com/
Did a copy and past from one of the sites ; 50L = 13G
There are many different sizes and styles of aquaria to choose from. An uncrowded aquarium will help you to observe the fascinating habits of your seahorses.
The minimum size should be about 50L (see individual species pages), which will safely house two seahorses up to about 14cm in length. Larger fish should be given a minimum of 80L per pair. However a larger tank (e.g. 150L) will allow you flexibility in interior design, while giving the seahorses room to explore their habitat. Deeper tanks are desirable if you intend to breed your seahorses (see Breeding).
The larger your tank, the easier it is to maintain optimum water quality