Reading up a bit on why this happens, its seems as the leaves are not turning yellow, this could be a sign of potassium deficiency, is that a fair assumption?
Buy yourself a cheap white plastic ruler and dip it in the water behind the DC so you can get a better look at the colour or maybe buy one of the white plastic dc's. Not as nice to look at in the tank but you get a better reading. The fluval one is quite good.
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Or it could be the plants settling in as discussed earlier. Two weeks is nothing in plant terms. You need to run your set up for a month or two for any issues to rear their heads I would say.
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