I probably won't be much help but we have a 4ft & 3ft tank with LEDs. 20w is the 4ft & 15w for the 3ft. We have anubias on driftwood & amazon sword & all is going great, touch wood.
http://www.ebay.com.au/itm/Led-aquarium-light-with-Moon-light-120cm-4ft-/130799421303?pt=AU_Pet_Supplies&hash=item1e7440c777&_uhb=1
We've got a 4ft & 3ft from here & touch wood, no issues what so ever.