That Hornwort is a good test. That stuff with decent lighting and nutrients Ive seen grow up to 4" per day and it grows way bushier. I would say you have enough nutrients in the tank to keep the plants alive and healthy as it seems you have pretty low requirement plants, but not enough to get any real growth. Using any ferts at all? Seachem Flourish and Excel can go a long way and root tabs for your stem plants...