You don't have to use a pH controller. That's an extra cost and more like a luxury co2 system item.
If you were worried about the pH going even lower, you could add some crushed coral, like mg suggested.
What kind of plants do you have and how often are you feeding your fish? You shouldn't have too much of a thread algae problem in low light IMO...so I'm wondering what could be causing yours.
A good way to get rid of hair, thread, staghorn algaes is Flourish Excel. Spot dose the algae with a syringe. I'd do this after trying to remove what you can. You can use 2-3 times the dose on the bottle. That has killed hair algae for me in the past.