Well, a sponge like a cleaning sponge is too thick for anything but mechanical, but a more pourous one like a scrubbie can be and are used extensively here.
That's cool. I really like the lava rock idea. You have any idea if that affects the ph? There are some sponges that look absolutely perfect for this as well so I'll have to ask around and see what people say
sure u can submerge them i had them submerged for a while worked for me but i had my baffles done a little differently for optimal contact time then i switched over to both a wet dry and submerged ceramics