ideas for 15 gal isopod tank

I have set up a tank for two species of isopod I found in my backyard. They are doing really well and breeding. The tank has layers of gravel, eco earth, and some kind of mulch that was included in a snake kit. I am looking for pet that would do well in here.
It can eat isopods but not wipe them out.
it must like humidity, I mist this tank every day
Ideally it would be OK at room temp

I'd like a tarantula but I have read that they need smaller tanks so they don't fall. Is this true?
Hermit crabs are way to complicated and will out grow it.
Maybe an american toad? Or a ribbon snake? Open to any ideas except centipedes. I quite like millipedes, though.

A picture of the setup would help.

What are the temperature variations of your setup throughout the year? Is the temp. stable or does it fluctuate?

In the winter it usually is between 66-70 degrees F and in the summer around 80. I would add a hide and a larger water dish when I decide on a main critter.
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