That's a nice tank size for a group of D. imitators, (or a close relative like intermedius,) perhaps 4 of them if you have some bromeliads on a background and plenty of other plant cover. A couple of unsexed leucomelas, galactonotus, auratus or P. terribilis would do well in there, but don't mix two species together. The tinctorius and azureus groups are more territorial and are better kept in mated pairs, which are more expensive to purchase. Except for the thumbnail imitator group, I wouldn't put more than two darts in a tank that size. The rule of thumb is 1 frog per 5 gallon capacity, but this is just generalizing, and in most cases, more space is better, especially with the larger darts.
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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