Sorry for the delay. P. terribilis are really good frogs in groups, and you could put about four in a 20 gallon setup. tinctorius are more territorial and are better in mated pairs, especially in a 20 gallon, which is small by their territorial standards. I love the mint green terribilis, although "green" seems to be a silly color to choose when one is dealing with the most colorful frogs on earth. There are a lot of green frogs, but none quite this same somewhat iridescent seafoam shade they become as adults. It's almost like they are sprinkled with powdered silver. (Froglets and juveniles aren't very impressive.) They are quite bold and uninhibited by your presence, and quickly learn where and when you feed them. They aren't the most active and exploring frogs among the darts, but they will use all levels of a 20 gallon tank. I have some of the orange ones too, but their color isn't as appealing to me as the mint green.
If you can afford them, a pair of D. azureus (closely related to the tinctorius, and probably really shouldn't have been separated from that species into a different one,) are beautiful, large, relatively bold and easy to keep. They are still relatively expensive, especially in mated pairs, which is strongly recommended.
Other good group frogs are Dendrobates leucomelas, (also among the least expensive), D. galactonotus (harder to get and more costly,) and D. imitator. The latter is a thumbnail frog, which is a bit intimidating at first, because they are so tiny. But they are hardy enough fellows and really active in groups.
Stick with the general rule of 1 frog per 5 gallons for the groupies, and 1 pair in 20 for the larger territorial guys, and it will be about right.
Remember that so called "arboreal" darts aren't necessarily strictly "arboreal." Most will use all levels of a tank that you can provide. And the terrestrial fellows will and do climb and use the higher reaches of a tank. They come from vast rainforests, where the distances aren't as confined or squashed into inches instead of meters.
Have fun with this. It is always great to have someone new to darts, especially someone with other herp experience. You will love them and become addicted.
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Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
7 D. imitator
6 D. leucomelas
4 D. pumilio Bastimentos
4 D. fantasticus
6 P. terribilis mint and organe
4 D. reticulatus
4 D. castaneoticus
2 D. azureus
4 P vittatus
2 P. lugubris