In a message dated 97-11-24 18:17:25 EST, Richard George wrote: << 3. Is the substrate point moot with regards to UG filters? If so, how can I compensate? >> George booth has a tank woth a UG filter listed on his web site. There is some good data there if he doesn't respond to your questions himself. His site is: http://www.frii.com/~booth/AquaticConcepts.htm Bob Dixon