I find myself quoting info from "The Krib" for the second time today. The following is a chart I've found to be most helpful. I hope you can find the info you need here.
COMMON SYMPTOMS OF NUTRIENT DEFICIENCY IN AQUATIC PLANTS
Element Leaves to first show deficiency Symptom
Nitrogen: Old, Leaves turn yellowish (*)
Phosphorus: Old, Premature leaf fall-off
Similar to nitrogen deficiency
Calcium: New, Damage and die off of growing points
Yellowish leaf edges
Magnesium: Old, Yellow spots (*)
Potassium: Old, Yellow areas, then withering of leaf edges and tips
Sulfur: New, Similar to nitrogen deficiency
Iron: New, Leaves turn yellow
Greenish nerves enclosing yellow leaf tissue
First seen in fast growing plants
Manganese: (**) Dead yellowish tissue between leaf nerves
Copper: (**) Dead leaf tips and withered edges
Zinc: Old, Yellowish areas between nerves, Starting at leaf tip and edges
Boron: New Dead shoot tips, new side shoots also die
Molybdenum: Old, Yellow spots between leaf nerves, then brownish areas along edges. Inhibited flowering
(*) The plants may also become reddish from the presence of the red pigment anthocyanin.
(**) Although Jacobsen does not differentiate between new and old leaves, David Whittacker reports from a hydoponics book that boron, calcium, copper, iron, manganese and sulfur are immobile elements and whose deficiencies affect new leaves.
How often are you doing water changes? This usually renews the nutrients your plants need.