There could be a number of things going on here but I will try and help you pinpoint the problem. Firstly, look closely at the leaves. Are the spots on the leaves brown or is it actually slightly purple? From the picture, I can't really tell but if it is purple, the problem might have to do with a Magnesium deficiency. When there is a Mg deficiency in your garden, the leaves tend to turn purple and curl under. Increase Mg by 100-200 ppms and the problem may fix itself. If your plants are in the veg stage, I would recommend using a Calcium-Magnesium liquid like Cal-Mag by Botanicare or CaliMagic by General Hydroponics. If your plants are flowering, I would use a Magnesium-Sulfur liquid like FloraNectar by General Hydroponics or Sweet by Botanicare. The reason being is that veg wants more Calcium and flower more Sulfur. I have provided a link to my favorite hydro store and they carry all the nutrients I have recommended. Note that if the plants get too cold, this can also be the cause of the problem. Also, too much light will cause the plants the curl under so be aware of that too. I hope this helps!

Here is a link that might be useful: Hydro Store

Its a calcium issue.
Purpling is phosphorus (P) related. Its commonly mistaken for a nute deficiency when cold temps inhibit P uptake.

also, check the underside of the leaves for something that looks like dust or pollen. could be mold.

also check for mites, sometimes they're too small to see without a magnifying glass, but you might see what looks like spiderweb strands on the underside of the leaves.