A 70 Gal. is same size as 90 but only 20 in. tall. I got mine for the depth and have been happy with it. I'm worried that if I go to a 6 foot long tank the length may squelchI the advantage of having a nice depth of field. My alternative would be to use something like a 4-foot long, 90 gallon tank - or maybe 2 :). Does anyone want to sway my opinion? How is lighting best arranged in a 6 foot long tank