I've got a pair of perc's in 10g QT right now. My filtration is a HOB with just a sponge in the filter compartment. The sponge has been in my DT for a month. I've got one of those hang on ammonia alert things, and today I noticed that it was beginning to change color. I tested with my test kit and the ammonia was very low, but detectable. I don't know why the sponge is not taking care of the NH4 but it's not. So I did a 4g PWC and then I put some LR rubble from my sump into the QT. This took care of the NH4. My thought is if I have treat the QT I will remove the rubble and in the mean time I will start to colonize another filter media in my DT to replace the sponge. My question is what media should I use to replace the sponge? My only choices for local stores are Petsmart and Petco (I know it stinks). Any suggestions?