There are charts online that show what you need to dose depending on what your plants are doing, check one of these out and buy a fert that is high in what ever you need. If you are happy with your plants don't dose anything, otherwise it will be a waste of money and you risk putting your fert/light/CO2 out of balance which could cause algae problems.
Have you tried checking your PH at different times of the day to see if it changes? There are really two type of stability with PH, short and long term. During the day the PH will swing for a number of reasons, plants using co2, fish breathing, etc, so if you are measuring the PH at the same time it might look like it is stable (because you have long term stability) but could be swinging quite a bit during the day. FYI I have an electronic PH meter in my marine tank, this shows around 8.0 in the morning before the lights come on, and sits on 8.3 when I get home in the evening.