Well the 20 gallon will do for a short time but really a 29 gallon is the bare minimum I would suggest for 1 pair of seahorses. I am sure they are small now but will most likely reach an adult size of about 6-8 inches tall maybe even more depending on species.
Seahorse require a taller tank instead of a longer one. Usually 2-3 times the horses adult height
Unfortunately from an ID standpoint black means nothing. Seahorses can and will change colour.
If you can get me a couple clear pictures of them I can help you ID them. This way we can set up a system best suited to them.
You can also look here and see if your's looks like any of them.
Seahorses
I am gonna narrow it down to 3 possible species for you for right now. But without seeing them I can't be certain. H. reidi, H. kuda or H. erectus.