AFAIK, there is no 48 inch bulb that is 33 watts. Perhaps you meant 32 watts (T8) or perhaps you meant 40 watts (T12)?
I'm surprised that with under 1wpg that your Hornwort grows at all. Is it floating (it would get more light up there, obviously).
I doubt that you'd be able to grow even the most low light tolerant plants with under 1wpg.