I have heat isuues in my grow room and it looks like heat to me? I am no expert though. A week is a long time before waterings and the pots should be very dry by then, could it be the pots are too big for the plants and the water feeding them is stale as they can't drink it fast enough? I had that prob too due to inexperience, but I put having the same problem as yours down to heat. Now my plant is bigger the problem has dissapeared.