Are they necessary?

[DanDrobates]

Isopod and springtails though not necessary are extremely beneficial. They eliminate a lot of the unwanted waste and they act as a supplemental food source. Once they are in the vivarium you’ll never even know they are there. It’s wise to seed a vivarium a few weeks before adding frogs. I seed all my vivariums and I see a definite benefit.

As for vivarium size an 18 by 18 is ok for a single smaller tinctorius but they do appreciate more floor space. A 24 wide would be a better fit. A thumbnail would do fine in that size.