I have also read peace lilies are not a good idea to fully submerge in betta tanks because they are not actual aquatic plants. I have some anubias which have done really well since I bought them a month ago....pretty low maintenance and very green and pretty =) Also, java moss and java fern are suppose to be good plants for betas as well. As a side note, maybe consider getting a floating log for your betta, too....mine loves his! It provides them with a "nook" to hide, sleep, and play. Good luck!