Duckweed is great at filtering out the light, if you want lights, and you only need a tiny bit to start. A few months ago I had only a dime-sized amount (came with some daphnia) and it's now a thick blanket across the entire surface. Both my axolotls 'hang out' near the surface during the day a lot more now, where it used to be a nighttime only activity. I think they feel safe with the duckweed above them!
I love having lights on my tank. You just have to plant it well enough that there are shady spots and give your axolotls hiding places to use if they get uncomfortable in the 'sun.'