We have an african tank and have loads of rocks. you shouldn't need to get silicone, we just put the biggest on the bottom and built up the rest on top. If you get rocks with a bit of a rough surface like Tufa (which will also help buffer up the Ph and Hardness) or Lava they can grip onto each other and will be plenty stable enough.wrs said:if there is alot on the bottem, adding more to the top should be fine, You should probebly contact people that have African tanks to see how to get them not to fall apart. Some people would recomend using aquarium sylicone, but with the Africans I wouldnt incase you need to change the territories around.