We used to use vermiculite, but now use 3-4 stacks of light diffuser which fit to the size of the tub (and water filled to about 1.5 stacks of diffuser). We then put saran-wrap across the top and then a secure lid. We crack open the saran-wrap/lid every once in a while to allow for some new air, and remove the saran wrap for the last week when they're due to hatch. The lids allow enough air in for breathing, but keep the humidity up. We haven't had to add water, and have had no dimpling eggs until the last week when they are due to pip.
Something about our incubator and using vermiculite just doesn't work. They got too dry, it was too messy, and annoying cleaning up after hatchling who climbed out into the vermiculite. Really loving the light diffuser all around.
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