Depends on the darts. There are some that are primarily terrestrial, so the horizontal option would work best and some that are both arboreal and terrestrial, so vertical would be the better choice. I'm far from an expert on darts, but i would say a pair in a 10g should be fine, especially if they are thumbnails. I could be wrong, so please don't take my advice as law......lol

Oddly enough, i downsized the amount of tanks i had this year, but they are creeping back up slowly.....lol