Tank is looking really good, well done! Great carpet you have going there. 840's are 4000Kelvin, the spectrum of the sun (at midday at the equator) is 6500K or thereabouts and is what a lot of people tend to get. You can go as high as 10,000K, 18,000K even. I've only gone as high as 10,000K and it looks nice over a planted tank. People have had great results with the higher kelvin bulbs but 865's (6500K) will do you pretty well.