The sponge they gave you is to seed whatever media you have in your filter right now. Put the media in your filter and start running it. I dont know how hardy convicts nor how big of a bioload they put off, but if the bacteria from the media the store gave you cant keep up with the bioload from the 2 convicts, your tank will go through a minicycle. If this actually happens, don't get the jar until your tank becomes cycled and stable. If this doesnt happen, the you can go ahead with getting the jar.
