Hello,
From my experience and some of my friends' :
- for terrestrial big-sized caudates (tiger salamander for example) : you may regularly add some dead leaves (preferably oak) above the substrate. The targeted effect is maintainig a high Carbon/Nitrogen ratio. Isopods and springtails can help to get rid of waste but they need a Carbon source, just like bacteria.
- for terrestrial small-sized caudates (small Plethodon species for example) : the substrate must provide some of the food for the caudates. One good solution is a mix 3/4 oak / 1/4 maple, in which springtails and isopods thrive well. One may take a stock of maple leaves in autumn because maple leaves disappear very fast in nature.
In these two cases, the first little animals (worms, springtails, isopods...) can be added with a rich substrate (garden compost for example) in the bottom of the tank.
- for aquatic caudates : adding some dead oak leaves in the water help managing nitrate levels (adding a Carbon source once again).
Of course, avoid surpopulating the tank.