If its just for trace elements then its totaly up to you if you want to continue adding it. Like I said, I just started adding some to mine, and while I haven't seen a real effect, I'm sure that if used at the right doseage, that it can't hurt anything.
A pH under 7 is way to low for any Rift Lake cichlids IMHO. I would get it up to 7.5 minimum. Above that would be great too. For Malawi, I would shoot for somewhere around 8.0 .
The driftwood may also be lowering your ph. Do you know the kH of your water? This is very important because this tells you what your buffering capacity of the water is. In other words how much acids can be added to the tank without it adjusting the pH. A high KH meens that it would take a lot of acids to lower it, but a low kH would meen that it wouldn't take much to do so. So if you have a low kH, and it sounds like you probably do, then the driftwood my very well be lowering the pH.
There are several manufactured buffers that will raise the kH and pH. But you can easily make your own much much cheaper. But it is very important to note that when you buffer a tank, you have to be consistant with it. My water is naturaly very soft with a low kH. I did an experiment a while back to see how quickly the kH would lower if left alone, meening without adding water or more buffers. In three weeks time my kH went from 20+, all the way down to 3. So you see how vital it is to change that water every week and add the same amount of buffers to keep the tank stable.
Its not difficult to buffer a tank and keep it where it needs to be. Like I said, consitancy is the key.