Don't bother with the waterfall for your first dart tank, try it for your second or third.
Springtails are not dusted. They are placed in the substrate and allowed to grow there and eat the rotting vegetation and be eaten as snacks by the thumbs.
You dust the fruit flies as follows
Almost every feeding with a 1:1:1 mix of herpavite/reptical/Repashy ICB (the Repashy is optional, I like it though)
Once every other week dust with only superpig
Once every other week dust with only dry vitamin a supplement (on the week you don't dust with Superpig)
That way once a week they get vitamin a but they get their regular all around vitamins and calcium most of the time.