My personal opinion, I wouldn't use gravel as a substrate. It holds waste and makes cleanup a bit more tedious. For larvae, I usually go bare bottom with a cluster of java moss for them to hide in. If you keep up with water changes, I wouldn't even use a bubbler or air stone. Too much current can be bad for them. In the wild, spotted salamanders migrate to vernal pools to breed and the larvae hatch in these pools. Vernal pools are still bodies of water that form from rain and melted snow.
Daphnia is great. Baby brine shrimp is good too, but you'll need to keep up with more water changes since they'll die in fresh water.
Black worms are ways great as well.