two possibilites:
your water has a high calcium level that's acting as a buffer and pulling the pH back up.
you plants (yes they're all consuming nutrients if they're growing) are depleting the more acidifying nutrients more quickly than the other nutrients. Nitrogen in particular.
In reality, it's probably a combination of both. while 6.7 is not ideal, nothing is blocked from the plants at that level. why don't you just let it be and monitor it for a few days. See how the pH fluctuates and how the plants look. so often with hydro, we see something out of whack from our expectations and are very quick to correct it. sometimes a wait and observe approach is more warranted.
what is the pH of the plain water and is it a well or municipal source? if municipal, your provider should be able to provide you with a water report that will tell you what is in the water.

I agree it's probably the water he started with that is moving the ph. An easy way to tell would be to use bottled water next time. It's about 60 cents a gallon near me, so I think the cost to do it once is worth the knowledge. If that solves the problem, he can start shopping for water filters.

I bought a reverse osmosis system to eliminate water issues. I live in southern california and the water is REALLY bad here. hardwater buildup on everything.
the system cost a few hundred bucks, but I don't have to lug water bottles and when the cost is averaged over the life of the system it will be pennies per gallon.

I am thinking about purchasing an RO system, but like Grizz said the pH is tending to rise a little less everyday now that I am leaving it be. We will see what tomorrow brings. If its still drifting I may put the HPA system on hold and invest in an RO system instead.

Are you on well or municipal water?

Municipal water. I live in the Ozarks, or in other words on top of the giant Ozark Plateau that is riddled with cave systems and limestone. Right out the tap, though, the water pH is about 7.3, I actually expected higher.

Here is the municipal water report in .pdf form

Here is a link that might be useful: CityUtilities Water Report

ROs are great, but you still have to replace the sump filters as well as the RO membrane after enough use. Take a look at the prices and availability of those when you shop for RO filters. They are a highly profitable item for the store and the manufacturer. Replacing the three filters in a system will cost just about as much as buying a new system altogether.

One trick with a home RO - typically they have two carbon filters in separate in-line sumps. When your RO stops putting out water, the instructions will tell you to replace both carbon filters. They only come in 2-packs. What they don't tell you is that the first filter the tap water hits will clog up much faster than the second. You can just replace the first filter and it fixes the problem. I would still replace the second filter every other time.