On my 125g African cichlid tank I have two 36" bulbs spaning the 6' tank across the front, and a two bulb, 4' fixture behind those fixtures. I use 3 differant kinds of 18,000k bulbs to cover differant spectrums and the fish colors look awesome. The darker top, back corners, caused by the 4' light over the 6' tank, helps de-emphasizing the overflow and inlets and returns.