Looks like a Magnesium deficiency to me. When the edges are curling up like that it's a clear indicator. Try adding 1/2 - 1 TBSP per gallon of epsom salts to water. This should correct the problem. Dissolving the salts in warm water is the easiest way.
You can also try Cal-mag if you like. Your local hydro shop should carry it.
Good Luck!