Problem with my anacharis

I have this great anacharis plant and I love it because it grows like crazy and gives my fish a great hiding place and diffused lighting. My problem is that my plans seems to be turning brown at the bottom. Could this be a lighting problem? I use Sechem fert when I do a water change and all my plants seem in great shape. I don't use CO2 or anything else.

Any advice?

It's a 25g lighting is a neon but I have no idea how many volts. It came with the tank that's 2nd hand.