Darts actually CAN swim, but they do need an easy egress if you have a pond, because they swim only when they fall in by accident. I have a couple of paludariums that combine aquarium/dart terrarium. I put slanted logs (mountain mahogany) from each section of aquarium to land. The frogs don't fall in often once they're used to the set-up, but when I first put them in there, they fell in quite a lot-- swimming swiftly to the logs.
Saurian is an excellent place to get frogs, and his shipping is the best. You can expect to get them well within 24 hours. In several years, we have had only one bad experience where his entire shipments for that day got misdirected and all but one arrived three days late and DOA. It was a sad experience for both of us, but Patrick guarantees live delivery and takes any such losses by replacing the frogs with no hassle. I have also gotten a few from Black Jungle, and these have been satisfactory and arrived in good health. I have no experience with the others. Patrick will also give you follow-up advice.
The only other different advice I would give would be to also quarantine adult frogs for awhile, but not in a sweater box. You can keep them together for a couple of weeks in a ten gallon tank with just paper towels or I like a cocoas fiber substrate, packed tightly, and maybe a few plant cuttings. Put the cups they are shipped in on their sides. These will contain leaves and give them a place to feel secure. This is just so you can keep track of them easily, make sure they are all eating and get aquainted. In a small quarantine tank, you can identify individual frogs by their subtle markings. This is useful for the future.
As an aside, both D. leucomelas and D. galactonotus get along in groups regardless of sex, if this is a concern. It is difficult, if not impossible to sex froglets, and you pay a lot more for sexed adult pairs. So if you are just beginning, want to enjoy a group, these two species (in my experience) are great. You might also consider Phylobates terribilis. They also get along in groups, although each establishes a separate resting territory.
Another thought: Dart frogs in general are escape artists, so you want to make your terrarium absolutely secure. I've discovered that if a fruit fly can escape, so can a thumbnail dart, and some of the larger species can get through openings you wouldn't think they could navigate. Some may leap past you when you open the cover to do maintenance, so fast, you may not see them until it's too late. Every one of my losses except one, has been from an escape of some sort. It's really hell to find a $150 frog the size of your thumbnail dried up on the floor and not be able to figure out how it got there.
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Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
5 D. imitator
6 D. leucomelas
4 D. pumilio Bastimentos
4 D. fantasticus
4 P. terribilis
4 D. reticulatus
4 D. castaneoticus