Curling, wavy leaves?

I have a couple of plants doing this at the new growth. What could be the issue?

A cannabis copper deficiency manifests itself as leaves curling back, lack of growth, and unusual coloring of the leaves. New young leaves may be dark and twisted, while other leaves may start showing signs of yellow or white. It is very unlikely that there is no copper available in your water or soil. Copper deficiencies generally crop up when there is a pH problem, so that the cannabis can't get to the copper even though it's there.
What you gonna do : Flush your system with clean, pH'd water that contains about half the correct amount of nutrients needed by the plant. Watch to make sure that the problem starts to clear up within a couple of days.

I'll give it a whirl and let you know if the problem is solved. My water (tap) doesn't register on a PPM meter, and there aren't any other signs of soil pH problems throughout the grow, just three or four plants with leaves like I showed.