Natural sea sponges are NOT bad for hermit crabs. In fact, they are a great food source.
“But your website says not use to use sponges!”
Yes. It does. However, there is some need for clarification here. The more accurate statement would be: Do not use sponges in the manner suggested by the pet store or packaging. Most pet stores and pet supply stores sell sponges as a multipurpose product.
First, as a means of drinking without drowning. That, in and of itself, is amusing considering most pet stores don’t provide enough water for a fly to drown in let alone a hermit crab. Hermit crabs in captivity are regularly seen swimming in deep pools, they just need some stable object to allow them to safely climb. Some spend hours underwater. Hermit crab drowning is not the major threat it is made out to be.
Second, as a means of creating humidity. Again, amusing, since most pet stores have open tanks with no lids. In order for sponges to provide enough humidity to make the habitat safe, the habitat would need to be packed full of wet sponges.
There are much better options to these particular ‘problems.
To learn more about making deep water pools safe for hermit crabs, please read A Guide to Land Hermit Crab Pools
To learn more about the correct way to create and maintain humidity, please read FAQ How do I create and maintain humidity in my crabitat?
But what if you really, really want that sponge to be IN the water? That’s your decision, so long as you know that bacteria grows in the sponge rapidly and it will need to be washed and sterilized daily. Bacteria in the hermit crab’s water is not safe.
Seriously though, just put the dry sponge on the substrate and enjoy watching your hermit crabs nibble away at it. Aaaah nirvana…