It's not the lights. Why are you doing 50% water changes every week (has the tank cycled?) You have no soil to speak of. Very little livestock. And you're emptying whatever nutrients that are left down the drain every week. Try planting something in a tiny container of soil and cap it with a tiny bit of gravel and place it in the bare corner and compare the growth. Trader Joe's sells a brand of k-cups that would do nicely in a betta tank.