i would really reccomend not putting a seahorse in that small of a tank. Seahorses are vertically oriented animals, and need to be able to travel a good distance vertically as well as horizontally. Placing one in that shallow of a tank could possibly lead to the animal having buyoyancy problems (air gets trapped inside the horse and the only way to cure it is to take a hypodermic needle, extract the air and then apply an antibiotic. Its not fun, trust me, i had to do it once, and although it was successful, i would never want to do it again) In the wild, seahorses occupay a fairly large area, and defend this area agaist other horses. If you are gonna do a horse tank i would really reccommend looking into like a 29 gallon tall tank. THis is what i have my horse set up in, and it works like a charm. everyone is really happy!
good luck
jon