Seahorses.

[rayjay]

Can't help with the 20 as I've never set up a tank that small other than my smaller dwarf tanks.
Be sure that you check out all the seahorse needs before you get too far in your planning for the 135.
i.e. - no stinging corals or anemone, corals that can live with the cooler temperature range of 68° to 74°F that give best chances of success with seahorses, preferably a species only tank, but if you ARE going to have other fish, realize there is a risk, even if you go by the tank mates list of acceptable stock.
No unprotected overflows or power heads, and no direct blast of water that can force them against anything and do them damage.