I have a TopFin powerfilter 60 on my 55 gal and I was wondering about my filter media. I use the typical carbon/floss cartridges that have to be replaced every month. I was wondering if I could use sponges instead of worrying about throwing out good bacterial colonies. If I did this, what would have to be in place of the carbon? Could I just buy a jug of carbon as a seperate media? With the seperate carbon, does it last longer, or does it have to be replaced too. How long does it stay good in the jug? Thanx in advance.