Saltwater is much different than fresh. Also, small tanks are more difficult than larger tanks to maintain, since the water quality is more unstable than larger tanks. You'll need to do more frequent testing and water changes. You may be tempted to overstock too quickly.
ALso, Seahorses can be challenging. Go to
www.seahorse.org to read up and ask lots of questions before you buy. 10-
gal is really way too small for a seahorse. You'd need a 20H to be successful with them, especially if you want more than one.
I had a 55-
gal fish-only tank a long time ago and had a seahorse in that, so this isn't my first one, and my setup is just temporary - until I upgrade in a couple months.
Good luck, do lots of research, ask lots of questions and be patient. Saltwater is a full-time hobby requiring dedication, but it's very rewarding.
Just my 2-cents.