Mineralized Topsoil. I purchased it from someone here on the forum. It was rich, dark, clean, compacted slightly moist soil with a separate supply of red clay, ash and dolomite. I prepared it with tank water and applied it per the MTS sticky by Aaron. I tossed the existing gravel in tank water to loosen the debris and put it back in the tank. I test this tank everyday because it's only been running since November and I know how fast things can go downhill with a small tank. It cycled in about a 6 weeks and before adding the soil, I had consistent readings of 0 ammonia. Yes, I did it in one day, it's just a 12 gallon tank. I don't clean the filter, that's really not something you should do. But you know, you did just remind me of something that might figure into this! I removed the activated carbon when I started dosing the ferts. I'm going to put in a new cartridge and see if that helps. If recall correctly, carbon doesn't remove ammonia though, but that's the only other variable so I'll see what happens.