Are the new auratus froglets or juveniles? I would imagine that they find the smaller food easier to capture and swallow at this time. They may be eating a few of the dusted flies when you aren't observing. The gnats and springtails are both good food for them, and they should do fine on them for awhile. Springtails in particular, aren't as deficient in calcium as fruit flies and crickets are, although the frogs will eventually need the D3 contained in the dusts to utilize it properly, unless you have some sort of UVB system set up. (This is difficult to do properly with dart frogs, so most of us depend upon the supplement dusting.) I do believe they will be switching as the established prey becomes more scarce.
In any event, I don't believe you need to worry about it at this time.
It is usually recommended that you keep all new frogs in a quarantine tank for the first few weeks. Some breeders even recommend separate small tanks or "plastic shoe boxes" for each frog, although if mine are siblings from the same breeder, I usually put them together in the same nursery. I use Critter keepers with the vents sealed with clear shipping tape, except for the top flap for feeding and misting. There are a couple of ways of setting these up, depending upon your concerns. The most simple is covering the floor space with wet paper towels and tossing in a few plant cuttings for hiding. This makes observing more simple, and if you are concerned with possible parasites, the collection of fecal samples is easy. On the downside, the towels have to be changed often, disturbing the frogs, and it is frankly a pain to do so without someone hopping past you, chancing an escape.
I usually use a base of an inch or so of either damp brown sphagnum moss, or damp coconut fiber, also with some plant cuttings tossed in, for my nursery tanks. A small deli cup or two on its side with a plant leaf and a little water in it, also insures frog comfort.
This gives you a chance to monitor each frog, make sure each is eating properly and behaving normally.
This is just a suggestion for the future, since separating your frogs at this time for quarantine might be more stressful to them than just leaving them alone where they are.
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Patty
Pahsimeroi, Idaho
D. auratus blue
D. galactonotus pumpkin orange splash back
D. imitator
D. leucomelas
D. pumilio Bastimentos
D. fantasticus
P. terribilis mint and organe
D. reticulatus
D. castaneoticus
D. azureus
P vittatus
P. lugubris