Yeah Hornwort is going to shed it's needles no matter what in higher temperatures. I'm not sure if it's just acclimating to your water chemistry which would probably be most logical or if it's due to just higher temps. I've never let it get that far so I don't personally know about that, I just know it sheds like crazy.
As for lighting, yeah LED's generally don't have a good enough PAR rating to grow plants except for if you got a Finnex Fugray, those are decent but they're also costly! Much more effecient to just get a CFL. You can do a desk top lamp or you can get one of those clamp on lights from Home Depot/WalMart : http://www.homedepot.com/p/Commercia...1#.Ue1Lh9I3szZ
and here's the smaller one: http://www.homedepot.com/p/Commercia...0354513#header
As for wattage, wattage is only how much electricity is being used, so that cannot define the strength of the bulb which is needed for low/med/high light. That's where PAR rating comes in (parabolic aluminized reactor rating) it's kind of lengthy to get into but basically it boils down to how good your light bulb is (fluorescent is greater than incandescent), how good of a reflector you have behind your light to reflect it into the water and then how deep your tank is. For 10 gallons and less it's easier to achieve higher lighting because the light doesn't have to penetrate as much! So you could easily get high lighting with a CFL on your tank ^_^
So anyway, you want to find a day light bulb, something with a Kelvin rating of 6,500K or right around there. 6,500K is what the sun is measure at on a clear, sunny, mid-day afternoon when the sun is at it's peek. Best time for growing plants so that's why you want one of those. They can be found at Home Depot too pretty cheap. I got a pack of two for four dollars
On my 3 gallon I actually went with a 5,000K bulb because the 6,500K was just too bright and it's been growing my plants well. But it's also a shallower tank than my 10 gallon, so you'd do best with a 6,500K bulb in there