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There are some potential problems with using sponges:
1. Dead sponge - If this is what you have in mind you'll need to consider that, in life, most sponges rely on toxins as their major form of defense. Usually not a big deal with live sponges since they only produce enough to deter predators. OTOH: After the sponge dies those toxins can be released into the water as the flesh decays. I suspect that the sponge will dissolve in salt water much faster than it would in use as a bath accessory.
2. Dead sponge - Do a Google image search on "Spicules". These are what's left after the sponge's flesh has dissolved. Depending of species they're made of CaCO3 or glass. I could be wrong but these things don't look like a specific against butt-burn to me!
3. Live sponge - The first problem is availability. It's harder than you might think to harvest sponges for the aquarium trade. Generally speaking, even momentary exposure to air is fatal to sponges. Of course you could do the 'whirl it up in a blender' trick which seems to get around the air exposure problem. Unfortunately there's little likelihood that when the sponge slurpee reassembles itself that it will be in a shape that you can use.
4. Live sponge - If you can successfully move a live and undamaged sponge to your tank you're then faced with the problem of how to feed it. This shouldn't be a deal breaker but you do need to have a plan in place before you start. BTW: A DSB of the sort described in the article I linked yesterday will go a long way toward supporting live sponges through its production of bacteria and protists.
why not just make a form out of resin (fiberglass resin etc). get play sand dig a mold and fill it with resin. stick in ballons and such for caves and pockets and presto. costom build aquascaping. cure for 3 weeks in high salinity water and your done
Using concrete or fiberglass to create rock formations is what I was originally going to do, but I didnt want to make a big butt burn factory. I am looking at big cuttles, and wanted to see if I could make something safer. I like big rockwork.
I think I am going to try dyeing natural sponges and see what they look like. If I dont like the final product, I will create man made rockwork.