The cheap Chinese led floodlights off eBay are awesome. I replaced my cfl with a floodlight led on my 10g I saw noticeable growth within a week. I was not seeing this kind of growth before with a cfl. I paid 12$ shipped. I had to wire my own plug on but that only took a few minutes.
For a 75g tank the 10w fixtures will not penetrate very well. I would suggest 3-4 20w fixtures or maybe a couple 30w fixtures.
Here's a couple threads on led floodlights
flood light LED's
Hope this helps