12 Volt - I rinse my sponges under the tap with hot water for as long as it takes to get all the crud out of them. Several minutes at least, up to 10 if it’s been a few weeks since I’ve cleaned them.
I am convinced that no problems occur because there is bacteria on all of the surfaces throughout the system, and even though I am killing off a small % when cleaning the sponge, their doubling time is so high they are replaced very quickly and no harm is done.
no doubt that IF you are indeed killing any beneficial bacteria, the rest of the populations in the tank handle the extra stress and you have no trouble.
but it may also be that your not really killing off any at all..its hard to say without doing some microbiology