HCA: Hermit Crab Association

I keep plexiglass sheets under my terrarium lid, everywhere except where my heat lamp sits, I think my temp and humidity levels would keep better if I had plexiglass under the whole lid, but am worried about the heat lamp sitting on a piece of it.

Will it melt? Will it release any gasses? Is it safe for the crabs?

That doesn't sound safe to me. I don't think a heat lamp should be sitting on anything other than maybe a screen. UTH would be a better idea, then you could seal the top all the way.

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Both. Plexi is very sensitive to heat, and plastic does release dangerous gasses when overheated.

Perhaps you could cut a hole in the area where the lamp would sit, and then put a piece of screen there. Many people make do it yourself lids.