The watts per gallon rule translates very nicely into watts per square foot and this turns out to be a much more accurate guideline. 2-4 wpg is thus equivalent to 20-40 watts per square foot of tank area. Steve Pushak Vancouver, BC, CANADA Visit "Steve's Aquatic Page" http://home.infinet.net/teban/ for LOTS of pics, tips and links for aquatic gardening!!!