Well, I know very, very little about sponge media, as I am a complete newbie myself.
However, just thinking about the basic rules of sterilising and bacteria and fungi, if it is the potential transfer of infection you are concerned with - ie you are trying to *kill* pathogens rather than preserve your nitrifyng bacteria - I'm not sure that sitting in freshly boiled water will do the job.
For boiling to sterilise and kill stuff, it generally has to be done for several minutes. I'd be concerned that if there were any particularly hardy, heat-resistent pathogens in there ( I *think* that fungal spores can be particularly immune to short blasts of heat) , that a bout in hot water might just encourage them to multiply in the warmth, rather than finish them off.
I am sure that you would kill off a significant proportion of stuff - but quite possibly not everything. I have no idea how resistant likely fish pathogens are, so I could be completely talking out of my bottom, but generally speaking, I would not regard something as being sterile or even necessarily clean, *simply* from having sat in a bath of hot-but-cooling water for a few minutes.
But what do I know - as I said, I'm a total fishy noob. I just have to clean stuff a lot
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