I run mine on (almost) a reverse photo period from the main tank. That can help stabilize any pH fluctuation from excess CO2. During the day, plants use CO2 and give off O2 (animals take in O2 and give off CO2). At night plants also give off CO2, which can drop the pH of the tank when the CO2 combines with H2O and becomes carbonic acid. By running the tank lights during the day & the fuge lights during the night, some people report that they don't have the pH drop they see when the entire system is dark at night.
The other thing to keep in mind is whether you grow cheato to primarily export nutrients, or to grow big and fast. There may be a correlation (more chaeto will bind more nitrate & phosphate) but remember the ultimate purpose. :lol: