My all time favourite is java moss. It seems to grow really fast in minimal lighting, which is prefect for the conditions you need to have an axolotl. My axy hides in there a lot of the time and it looks great in my opinion. Other than that, all I've managed to keep successfully are java ferns, but they get unsightly brown spots which sprout out little roots and leaves of baby java ferns. It doesn't look nice and they always create them! So I get a little annoyed with that, because I have so many planted now, that I have to throw them away as soon as I see them sprouting. They also appear to dye the sand brown/black even though I washed them throughly before planting them. And if I have to uproot them, they give off a sulphuric gas which smells bad and must be of questionable health for an axolotl. I personally wouldn't recommend them, but maybe I'm not giving them the correct care they need,but in that case, who wants a plant which needs lots of work?