Id have read this post in the past and can only remember some one saying to use sand as a substrate wich is a very good idea buy to be honest no substrate is even better. I have a mate who has been trying to keep his ammonia levels down in his rtc tank and has just not been able to doing huge water changes added more than twice the amount of filtration required but still no luck till today till i when around and removed all his sand subtrate from the tank did a water test and perfect tested the water the san was no sat in and the amonia tested 8ppm. It was also making his ph bad tank is now 7 water from sand 4, So hopefully he will have a very happy rtc from now on. Thought this was worth taking in to acount whith a rtc tank.