75 gallon, 18" deep...Single Bright or Double bright Marineland LED? I've read better reviews of single.

On each of my two saltwater 75 gallon tanks I have an Odyssea Quad T5 High Output light that has two 54 watt actinic and two 54 watt 10,000 K T5 bulbs. The fixture also has two blue LEDs built into it. The entire fixture was $100 including shipping on ebay. I really like it and would recommend it to others :)

I also use the Lithonia 32 watt 4 ft T8 shop light from Home Depot ($20) and its $10 Walmart equivalent with Daylight Deluxe T8 bulbs. It works really well; the bulbs seem to have a nice spectrum for growing plants.

Lithonia shop light: http://www.homedepot.com/h_d1/N-5yc1v/R ... reId=10051

Daylight Deluxe (this link is to a 10 pack, but they are also sold in the store for $10 for two bulbs): http://www.homedepot.com/h_d1/N-5yc1v/R ... reId=10051

Pictures of it on my tank:
http://img.photobucket.com/albums/v415/ ... /012-3.jpg
http://img.photobucket.com/albums/v415/ ... /018-4.jpg
http://img.photobucket.com/albums/v415/ ... /017-3.jpg
http://img.photobucket.com/albums/v415/ ... /014-4.jpg
I put two up there so the fish can't jump out but usually only have one on at a time.

it depends what your looking to do. for me the more light the better but some fish like dimmer tanks (discus, piranhas, etc..) but if your going with live plants id go with the double atleast. the make a reef one that would probibly be even better plus it has a timer built in