i went with nine inches since it was between 8 and ten. am only keeping it on for a couple hours in the morning for a few days will slowly increase time.it made my pulsing xenias swell up so much they could hardly pulse the stupid xenias are shading my clams and my little sea hare just wonders around the glass and wont go on the rocks. on a happy note i got my protein skimmer for my aggresive tank. i managed to catch the damsels that were in that tank and move them after i tore up my rockwork to catch them the water settled down and i like the way the rockwork turned out except i cant find the rock that had one of my mushrooms on it, and i saw some really weird looking snail thing crawling around. anybody who thinks this hobby is relaxing is crazy.